The name of Humphrey Keeper, beyond his family and community in south-east London, may not be immediately familiar, but a sizable chunk of the planet has heard him sing.

Before 27 July, Humphrey was just a normal 11-year-old boy from Forest Hill, who liked playing football and computer games and was enjoying his last summer holidays before starting senior school. But as the world's attention turned to Stratford, expectant to see how Danny Boyle would launch London's Olympics, the director chose to hush the clamorous expectancy with a moment of genuinely memorable stillness.

Bradley Wiggins signaled the start of the ceremony by clanging an enormous bell, before Humphrey rose to his feet and, unaccompanied, began to sing. "And did those feet, in ancient time, walk upon England's mountains green?"

He wasn't especially nervous, he says with the modest self-possession of a boy who has sung to a billion people. "The leader of our choir, she's quite good at tackling the nerves with the children. Because she made us go over it so many times we knew we were probably going to get the words right, so we were really confident."

He and the rest of the Dockhead choir, a church singing group from Southwark directed by Mag Shepherd, had been rehearsing Jerusalem in strictest secrecy for more than six months...

...His selection is all the more remarkable since Humphrey has never been formally trained to sing, and only joined the ceremony preparations when his friend Alex's mum, who knew Shepherd, mentioned she knew a boy who could really sing. "Yeah, I lucked out."

"I wouldn't know because I'm a parent and I'm biased, but I've always thought he had a special voice," says Humphrey's father, Lee, who runs a design studio. "I don't brag about it because it's just Humphrey, and he's got lots of things that are special about him, for us."

One of those things – though not, as Humphrey describes it, a particularly remarkable characteristic – is that he only has one hand, having been born with his left arm missing below the elbow. "It's never anything that has held him back at all," says Lee. "It was a shock when he was born, but ever since then, he's just been Humphrey. We can't see him as anything else."

Humphrey agrees his arm doesn't stop him doing what he wants to do. "I enjoy rock climbing and things like that. I just use my legs a lot."

Click here for a video of the opening ceremony of the 2012 London Summer Olympics.