If he wants to quit, let him. If he stays, tell him that he could only do it if he intends to be a postive influence. A team is comprised of all they players, and they all contribute whether they are in the game or not.
Casey, the coach ought to decide who plays and who doesn't play. Perhaps this boy simply is not good enough -- in the coach's view -- to make the lineup. If that is enough to get the attention of the AD, then thank the Lord I am nowhere near this place.