I would encourage him to stick with it because when he joined he made a commitment to his team. In life we sometimes must do things that we just don't like for whatever reason. However if he does not want to rejoin when it comes time to decide then I would give him the option to stop playing baseball.
I am gonna share my experience, it is something to think about it. His disliking baseball may just be the current situation. In junior high I was in marching band and other musical activities. I hated it, would come home crying every night, begged my parents to quit marching and all music activities because of this one girl. This probably went on for an entire season or longer. Things would improve and then they would get worse again.
This one girl who kept shouting out instructions of what we were supposed to do which kept distracting me and several others instead of helping me and was making me not recall what I was supposed to do. Let's just say at the time I hated it. A few of us went to the band and choral directors and talked to the director, he told her to stop because it was making things hard for others and that she was helping and not hurting the group. Sure enough people were doing better without her "help".
Well I ended up staying in Marching band, choral groups, musicals, etc in high school and through college and loved it. This girl chilled out and stopped giving her directions and things improved for me and others, she also eventually dropped out of some of the activities. Before you tell him he can quit I would definitely want to know why. If you need to talk to coaches or whatever I would do so. Had I quit because of this girl I would not have had many awesome experiences.
Had my parents not encouraged me to stick with these musical activities, I would not have had the opportunity to perform in WDW, at a Jacksonville Jaguars game, at the Lincoln memorial in DC, Superdome, watch Broadway shows in NYC, go to Illinois, Mississippi, Wisconsin, Virginia, and I could go on.
He just might not like the activity but there could also be more to it, than what meets the eye.