My son plays hockey and I was worried as well about the exertion and injury (GI also said he could do whatever he felt capable of doing). When he was diagnosed, he was playing on two hockey teams (so, on ice, probably, 5+ times per week) plus was in two phys.ed. classes at school (so, 2+ hours every day)!
I spoke with all his coaches and teachers, explained his diagnosis and requested that they both watch out for him and respect if he needed to rest. Also, stressed to Stephen that he had to listen to his body and not push himself if he felt something felt 'off'.
We had only one issue - he did feel some pain with long distance 5K running (but not when running 'sprints', such as in a soccer game, etc.)... unfortunately, one teacher obviously just didn't 'get' it and, when S would walk instead of running, teacher made comments like 'no pain, no gain' :ymad: but S told the teacher it was crohns pain and he was going to walk. Teacher let it go but it did annoy me that, after our conversation, he still made S feel that he had to justify himself.
It did take S a few weeks to get back to his pre-diagnosis strength and stamina (had to regain weight, etc.) but, after a gradual 'rebuild' he was back to normal and actually, the hockey season following diagnosis, was his best ever.
As for injuries, unrelated to crohns, he did end up with a separated shoulder and a knee injury but, neither affected his crohns nor did I see that crohns affected his recoveries at all.
I think most important is that your son understand that he has to listen to his body. I stressed to S that if he ignored cues to avoid missing a game or practice, he would end up missing a lot more if he ended up flaring due to overexertion.