Based on the fact that he doesn’t want to play beyond high school, I would not keep playing summer travel ball. The cost and time commitment simply isn’t worth it. I know this is going to go against the grain but just pull him from the team. Yes, let him quit, that’s a life lesson too. Sometimes you find yourself in a situation that makes you miserable and isn’t leading anywhere and it’s ok to just stop. It’s ok to stop being miserable, stop wasting money, and stop wasting time. Recognizing when you are in over your head and making a decisive move toward self preservation is a valuable life skill.
If he still wants to play the game but not at the highest level find a recreational league and keep enjoying the game. Nothin wrong with that at all.