I think it's more about the fact that it's not fun to lose and the way it's happening. He puts a lot on himself. He feels that he needs to be perfect and if he's perfect they have a chance to complete. Being good, the coach relies heavily on him to be perfect.... I do think the weight of the team is on his shoulders. I think this tension makes playing not fun.
IMO, not a valid reason to not play HS ball. What will he do if the college team he commits to ends up being "not good".
As others have said the college coaches will be wanting updates from his HS coach from time to time. If he tells a college coach he stopped playing because the team "was not good" it is going to work against him. In addition there is the possibility the college guys find out he was playing HS for a while and stopped. They may decide that want to put a call into the HS coach to verify your sons story.
At the end of the day, your sons situation is not unique. Many high level players play for HS teams that are not good. They find a way to suck it up and deal with it. Your son needs to find it within himself to get into a mindset that supports his HS team.