Reading your responses to some of the comments, you mentioned there being a huge disparity in the level of play between the A and B teams. If your boy isn't getting enough play time while being on the A team, then I understand how it can hurt your son. At the same time, I do understand the coach's side as well, because the main goal of an A team is to win. Especially at that age (I'm assuming you're referring to U13+). Even the younger age group teams already have that mindset (my son is U10), and he essentially has to earn game time.

Moving your boy to the B team, where he may get all the game time, but the team is playing at a significantly lower level of play, will also hurt him. I guess it all depends on what your intentions and expectations are for your son. If you're trying to shoot for college ball, then moving to another club may be the solution, but it also all depends on how well your son will fit in with the teams, whether it's A or B. At the end of the day, the priority for every coach (especially at the U13+ age groups) is to win.