Probably just gay or bi, not trans, though the film didn't really get into that kind of thing. At some point between getting into ballet school, and staring in swan lake, there's a fair chance he had some kind of a relationship with another guy or at least a major crush on one. If you have the 'bump' that gives you both the desire and ability to dance like that, then you've also got the 'bump' that makes guys seem quite attractive, whether you choose to act on it or not. Once the parents have got over the hump of accepting that their son would rather be a dancer than a boxer, and decided to support them, and made all the sacrifices necessary to get them good enough to get into such a prestigious school, they probably aren't that worried any more about their sexuality either. The guy's successfully escaped from his roots, he lives in London around other people with similar personalities and interests, he can pretty much get up to whatever he wants. ;-)
"Well, we're really proud of who you turned out to be son, but still, best not bring the boyfriend home and upset the grandparents or neighbors."
That's the plotline they save up for the sequal, or something. ;-)