Solid and thoughtful guidance about how to think about a kid who MIGHT want to quit baseball:

Should these parents let their son quit baseball?

Our younger son got super frustrated and angry a few times last season and said he wanted to quit. We listened to him but let him know he at least needed to honor the commitment through the season and then he could make his own decision about it at the end of the season. It passed way before the end -- I think part of it was he just wanted to express his anger in a way that would have some true weight to it.