I think the coach can do whatever he wants. There might be more involved than just the injury. If a player is a starter or one of the best on the team, they won’t be cut like this. Was your son on the bubble anyway? Was he second string or spent a lot of time on the bench (or the equivalent for his sport)? Maybe there was a very strong freshman class and he wasn’t going to make the team anyway. There are never any guarantees.