He should make sure he is getting sufficient carbs in his daily diet, and also after games to replace muscle glycogen. The first 45 minutes after exercise is very important (see what-should-i-consume-after-exercise), but in general, he should make sure he has sufficient fuel available for exercise from his entire diet (not just before and after). That will help reduce the crashes.
What is his diet like? He needs to taking his diet very seriously to compete at his best.
Not sure how he is doing it, but I would suggest not eating like a diabetic, but rather eating like an athlete, and making the diabetes work around it. It is possible to do that. Personally I never let diabetes influence my meal, my first consideration is eating what I need to help me run my best.
I would wait until after he has made the team. It is not something that should influence the coach’s decision, but it is possible it would. So I think he should wait until after he is on the team.
And I wouldn’t make it like it’s any big deal or something for the coach to worry about, just a BTW mention. “By-the-way, I am a diabetic, but it’s well-controlled, it won’t get in the way. You will never know it, I’m here to play ball.”
Then he needs to make sure it doesn’t get in the way. And don’t bother the coach with a litany of items of what to do if I am high, what to do if I am low, etc. The team’s athletic trainer should be able to help him with any specific diabetes things he needs.