You shouldn't have confronted him. It sounds like the coach could have handled it better but was within bounds. When a coach tells a player to do something he has a right to expect them to do what they are told. The player had the option of telling the coach what the problem was, but simply ignoring the coach was wrong. This is a chance for you to help your son learn a lesson. He needs to learn that he either does what the coach tells him or if he can't that he has to fess up to the problem is that is keeping him from doing it.