to the immediate PP
Just to play devil's advocate, (or in this case, your son's advocate)
Remember, it's possible that your son is not lying, but that possibly the coach misunderstood what he said, or (less likely, but still entirely possible) the coach is lying.
It's important to teach your child that he can always count on you. If he has real issues with the coach, I'd find another. It may be a red flag.