So, as I read this he is going to miss voluntary workouts as he does his winter sport. As others have pointed out this is probably not something new to the baseball coach. As long as he is communicating his schedule conflict and desire to play on the high school baseball team, I can't think of anything else he needs to be doing except keeping his grades up (which he is doing). If he has a great relationship with his swimming coach, it wouldn't be a bad idea to ask him to give the baseball coach a call to tell him what a hard worker he is, great teammate & student. It can't hurt.
Welcome to the Board, and best of luck to your son at tryouts. We've all been there, and we know what you are going through. So don't be shy and wait so long to post any questions you might have.