My advice -
1. have your son go to the swim coach and ask if he had a problem with him participating in the baseball workouts AS LONG AS THEY DO NOT CAUSE HIM TO MISS ANYTHING WITH SWIMMING
2. if swim coach doesn't have an issue with it then talk to the baseball coach to work something out. This shows a lot of maturity on your son's part to identify a problem and take positive steps to finding a solution. I know if I had a kid come up to me with this situation and solution it would make a VERY HUGE impression in a good way
Someone mentioned why swim if you don't care about competing and overall I agree with that but make sure your school doesn't have a rule about quitting a team in order to participate with another sport. At my school if you quit a sport you cannot participate in any other sport until the sport you quit has completed their regular season. The head coach of the sport that was quit can release the student but it's completely their decision.
Like someone else said above - this isn't a serious situation because your son is not the first to have this happen to but it doesn't make worrying any less. I have a catcher who is playing basketball that could REALLY use our workouts. He's a sophomore that has a legitimate chance to by our varsity catcher but he has a few things he needs to work on. Right before basketball started he was on the fence about playing. I told him he needs to play basketball and I would be disappointed if he didn't. He loves playing and he could be a big contributor on varsity when he's a junior. I'm not worried about it - we will get him caught up and see how it goes. Another factor is I'm also the athletic director so I can't quite tell him to not to play basketball when I preach all our sports need to find ways to share kids.
Glad to have you on board