Son's story: keewartson always played multiple sports. Basketball was his main secondary sport. He played 3 years for middle school and the team won the championship his 8th grade year. He played JV basketball in 9th grade. With all the running he was in great 'running shape', but lost weight. He started a sport specific strength program in 9th grade also, and time was precious keeping up with school work, too.
So, he decided not to try out for basketball in 10th grade and really work hard on the strength training and gaining a few pounds that fall/winter. He knew he wouldn't be recruited to play college basketball, and wanted to focus on baseball instead and the possibility of playing in college.
That 10th grade HS season he started to get serious looks. Summer after 10th grade he committed. He was a SS that was the same height and weight as your son.
My suggestion: see how the baseball tryout goes and hopefully he will make the team and make an impact. Next fall, decide if swimming was worth it. If he makes an impact, maybe the coach will understand the predicament for the next several years. If he doesn't make the team or make an impact, make some changes.
The followup: My son decided to play rec basketball the rest of his years in high school instead of for his HS team. He was playing with a bunch of buddies, playing against a bunch of athletes from around our area that were in his same predicament, and having a ball and continuing to keep in shape but without all the practices that go along with being on a team. Maybe your son can do the same with swimming.... to keep in shape....but maybe not compete?
Keep us posted! Good luck!