I'm guessing part of the problem has been generated by your son. Then how he carries himself bothered the coach. Your son should politely approach the coach and have a talk with him. Your son will need to understand who the person of power is (it's not him) before having this talk. The situation may not be hard to fix. The coach may respect your son manning up and addressing the situation.
I'm sure your son would rather be playing. But regardless of what happens he (or you) shouldn't bad mouth the coach. If asked, "I would prefer not to discuss it." It would be better not to have a red flag senior year. It may be harmless. It may not. You never know. But the easy fix is fix it now.