His name has been transformed almost overnight from that of a pedestrian everyman to a burgeoning legend, evoking the likes of Sam Cooke and James Brown and, by default, their old-school appeal, from a time when a singer was just a man in a suit with a great voice, crooning about love and loss. As I walk down Sunset Boulevard, one week before the Grammy Awards, Sam Smith is everywhere – staring out from the cover of Rolling Stone in the 7-Eleven, like a doe-eyed George Michael; his album stacked next to the cash register at Starbucks; a guest on The Ellen DeGeneres Show; on a billboard, dwarfing passing Chevies, his eyes and quiff lowered to the ground, in the grip of melancholy.

In just two years, this 22-year-old from Cambridgeshire has gone from being a bathroom attendant in a city bar to that lofty superlative “the biggest male artist in the world’’.

His album about unrequited love, In the Lonely Hour, entered the US Billboard chart at No.2 in November (the highest ever for a British debut singer), was at No.1 in both the UK and US album charts and sold more than 2.5 million copies globally.

A No.1 album for iTunes in 44 countries, including Australia, In the Lonely Hour went double platinum in Australia, UK and Ireland and platinum in the US, Canada and New Zealand.

In Australia, the album has stayed in the Top 20 for 37 weeks since its release and shows no signs of disappearing. His debut single, Stay With Me, sold more than 3.5 million copies and after our meeting early this month, he won four Grammy Awards – Best New Artist, Record of the Year, Song of the Year for Stay With Me and Best Pop Vocal Album for In the Lonely Hour. He entered this week’s Brit Awards with five nominations and as hot favourite to clean up. All this comes without the hype of The X Factor or a Brit School education (Brit School is one of the UK’s most prestigious schools for performing arts and technology). Physically, Smith is unremarkable. When I bump into him backstage at the Forum, a neo-Greek amphitheatre in LA where he is playing tonight, I mistake him, in his white T-shirt and jeans, for a sound engineer.

He is 1.88m tall and stocky, a bulky package of congenial warmth. His face is expressive and there is something in his eyes – intense, looking likely to brim at any moment – that draws you in.

His music is neither groundbreaking nor cool. It cuts across age barriers with raw emotion. To really understand the effect of Smith, you need to see him – rather, hear him – live. He is all about the voice.

Each of his album hits – Stay With Me, Lay Me Down, I’m Not the Only One – is punctuated with autobiographical outpourings:

“I wrote this album because I fell in love with a straight guy last year and he didn’t love me back ... Just before I wrote Good Thing, I deleted his number ... I wrote Money on My Mind because someone in the music industry pissed me off ...’’

He launches into a soul version of La La La, the Naughty Boy track on which he featured, which went to No. 1 in May 2013 and paved the way, along with his breakout vocals on Disclosure’s Latch in October 2012, for the release of his solo album. Then he sings a cover of My Funny Valentine. Somehow, all of this is delivered without a whiff of cheesiness.

The next morning, Smith and I sit with our feet dangling in the pool of the Ramada Plaza hotel, the sky above us unsure whether it wants to rain or shine, reflecting Smith’s ambivalent mood. He is anxious about the upcoming ceremonies.

“I don’t like the idea that if we came away with nothing we’d be disappointed because, in my eyes, we’ve already won,’’ says Smith, who prefers to say “we’’, as he has three managers.

“I’ve sold so many records, played so many arenas.’’

So he’s not feeling on top of the world?

“It’s really up and down, you know. I haven’t stopped for two years. There are moments when I love it and there are moments when I ... I wanted this life for so long, for a range of things. My insecurities. The need for music to be my therapy. The attention. The glamour. Getting the love. If all you want is fame and there is no creativity behind that want, you’re f---ed. This world is a monster.’’

Smith says that whatever happens at the Grammys, he’s skipping the after parties and going to McDonald’s. He usually ends up having a burger after awards ceremonies and they’ve been almost back-to-back since he won the 2014 Critics’ Choice award at the Brits and gongs at the Mobos and the American Music Awards. That’s a lot of burgers.

He is now on a carb-free, dairy-free diet. I’m disappointed that he’s not more righteous about his right to be a size “normal’’ – in the words of Adele, with whom he has been consistently compared, he is “making music for the ears not the eyes’’.

He is not remotely fat, just not toned and packaged to within an inch of his life.

“I do want to be skinny. I’m trying.” He giggles, as we prod each other like two kids in a tickle fight.

Smith is not the sombre, introverted character he portrays in his music. He is gentle but cheeky, like a giant puppy prone to fits of depression. He confesses he sometimes wishes he had abs like Justin Bieber.

“Bieber and I had a sing-off in the toilets on the Ellen show a few days ago. I was doing my vocal warm-up and through the wall on the other side someone started mimicking me, so I started going higher and he was going higher, too. I didn’t know it was Bieber until he came up afterwards.’’

Smith has quickly attracted a celebrity following through the sheer quality of his voice: Beyonce compared it to “butter’’; Mary J Blige asked him to co-write songs on her new album, The London Sessions; most surreal of all, Lady Gaga commented on her ‘‘strange, visceral reaction’’ to his music.

Smith was given three days’ detention when he was 17, for wagging school to see Gaga at The O2 Arena (during the punishment he drew a dream board of his ambitions – a picture of himself with a number one record, another holding a Brit, another playing at The O2 – all of which have come true).

On the subject of his growing celebrity circle, he says: “Sometimes I have this horrible feeling that maybe people are more interested in what Kim Kardashian smells like than my music. She smells f---ing glorious, by the way. She’s like an angel. I was with her last night, her whole family came to my gig. I saw a video of them singing along. We’re all just people. Even Beyonce, when you meet her. You build this person up in your mind to be this deity. But they’re just people. Fame is a very warped thing.’’

Smith has already had some experience of the warpedness of that world. US radio heavyweight Howard Stern recently ranted on his show that Smith was: “An ugly motherf---er. He’s fat. Is he gay?’’

Smith says now: “People make comments about me all the time and it hurts me. I read it and I’m offended by it. I’m not going to pretend that I’m not. I’m hugely vulnerable. But I don’t want to have any barriers because that feeling of honesty and connection with my fans is just the best in the world.’’

Smith is a curious mix of fiercely driven and highly sentient.

“I’m very emotional, but I’ve also got a massive fire, a hunger for success, for my music to be heard, which comes from my mum.’’

He grew up in the village of Great Chishill in Cambridgeshire and is a perfect blend of the polarised qualities of his parents. His ambition comes from his mother, Kate, a former bank clerk who was talent-spotted for a trading job in the city, which afforded the family a swimming pool. He inherited his sensitive side from his father, Fred, a fruit and veg stall owner who retired as breadwinner to be househusband and look after Smith and younger sisters Lily and Mabel.

“I grew up in a female-dominated household and I relate more to women, but my Dad is also in touch with his feminine side. He gave me an emotional poem before I went on stage at Madison Square Garden recently, about being yourself. I just burst into tears.’’

His pro-woman upbringing manifested itself in his love of power divas like Chaka Khan, Mariah Carey and Beyonce from early on. His parents took his ambition to become a singer, aged eight, seriously and enrolled him in lessons. He attributes his range to obsessively practising scales and the desire to bend his voice to the heights of his female musical heroines.

His father dedicated his life to his son’s career, ferrying him between classes and taking courses to become a personal trainer to work on his son’s physique. (Smith now has his own bodyguard/trainer, Adi, and says he feels a little guilty that he’s taken away his Dad’s role). It’s still painful for him that, in the newspapers at least, a public blow to his mother’s career was attributed to his young musical ambitions. In 2008, she was sacked by her city firm for “gross misconduct’’ and when she sued them for £1.5 million, The Daily Mail ran the story under the headline: “City banker is ‘sacked for spending too much company time on son’s pop dream’.”

“She was treated so unfairly,’’ Smith says. “But what it did was really give me the hunger to look after her. She deserves the world and that’s what I’m trying to do, give it to her.’’

The subsequent loss of family friends, who disappeared once the money dwindled, has prepped him, he says, for the ways of the music industry. “I’m prepared for this all to fail,’’ he laughs. “I think, if this all goes down the shitter, I can still say I once sold five million records.’’

As the only openly gay teenager in Great Chishill, he was often subjected to abuse. “I felt isolated. I was called ‘faggot’ many times. I worked part-time in a shop. There was a man in the village who had a massive issue with me being gay and didn’t want me serving him.’’

Things got worse when he moved to London at 18.

“I got punched in the neck in the street. I used to wear a bit of make-up. I had pink earphones on and was talking really loud. It was definitely a homophobic attack.’’

Although Smith had come out at an early age, he had no real sexual experiences until he left home.

“To the age of 16, I’d never met another gay person. When I moved to London, the gay scene was a real eye-opener. Some things were amazing but other things are very dark.’’

He had a lot of one-night stands but “wasn’t very good at them’’. He tended to fall in love with straight men – the first was a boy at school, two years his senior, and later came his unrequited love for a married man he wrote about in In the Lonely Hour.

“I’ve stopped falling for straight men now. After the last one, there’s no way it’s going to happen again. The record was therapy, closure. It saved me.’’

I warn Smith never to fall in love and live happily ever after.

“I know,’’ he chuckles. “I’ve written a song about another person recently. I split up with someone a few weeks ago.’’ He is referring to model Jonathan Zeizel, whom he met on the set of his video for Like I Can and had been dating over Christmas.

“Nothing dramatic happened. I made a mistake by posting pictures of us on Instagram and making it seem more serious than it was. I flew him to Australia and we had a nice time.’’

He tells me about nights on the town and skinny dipping.

“We just weren’t very compatible, but I’ve learned that I need to hold off before I start getting the public involved. I would say it was a relationship, but I still feel like I haven’t had a proper boyfriend yet.’’

Fred Smith recently chided his son for being too honest in the press. And I worry, too, about this talk of no barriers and giving all of himself away.

“My life is my life and things are going to happen and I’m going to document it and say how I feel. I don’t play a character, I’m just myself.

“I want to be a different type of pop star. I want to be a pop star who’s not photoshopped, who’s straight-on human ...

“Honesty is timeless. I’m just trying to make music that stands the test of time so that, in 400 years, when a little kid who’s gay listens to In the Lonely Hour, or my next record, he will be inspired.’’

Sam Smith plays Riverstage, Brisbane, April 25, frontiertouring.com/samsmith