We know one side of the story, and let's assume it's 100% accurate.
Some coaches have a hard time noticing progress. My two sons played for the same coach for four years. He seemingly saw in them, each year, how they played when he first saw them, and kept them on the bench, playing instead those players who looked good the first year, including his own son.
When they moved up to HS Ball and everyone had to try out for a coach who had not seen any of them before, my sons made the teams and three of the prior coach's favorites, including his son, did not.
Perhaps some time away, playing for a new coach, will rejuvenate your son's interest. If it doesn't, let him make his decision after educating him on the responsibility to accept all the consequences of that decision.