He has to have had some training, though. I haven't heard him sing, because I haven't watched BGT this year, but I've seen pictures and he stands correctly whilst singing.
I don't understand all this about his voice breaking though, it can't be controlled so yes, it can go at any time but that's true of any boy from about 11 upwards. Even though he's 14 sometimes they can go on longer, I was almost 15 (and anyway, you don't know when he was 14, it might only have been a few weeks ago). In any case, some boys can still sing when their voice is changing, on the other hand, others can't, it depends. But I'm sure there would be a plan IF he did win AND it did break before the show, he could mime, sing something else, or they could send someone else? It's irrelevant.
As for the stage presence thing, choristers are pretty much expected to remain perfectly still...