From your few paragraphs, it sounds like your S would be a great doctor, so why interfere with what he wants and what could be a good choice for him.
You should let two forces guide the recruiting process – the schools’ interest in your son as a swimmer and his college and life goals. Nothing else really matters. I always said that I would step in only if my kid was headed for a mistake. Majors can be a distraction, particularly for med school. Many may disagree, but I view college as brain training rather than as a trade school for a future job (with certain notable exceptions, like engineering).
So, make a very long list of possible colleges – dream big on the possibilities and include those that you think are “also rans.” Then start contacting the coaches with times (I have learned from this site that video may may be helpful for swimming so that a coach can assess room for improvement) and see what kind of traction your son gets. It could be that only D3s or D2s respond. Then you have let the process solve your concerns about time commitment (you might be surprised about the commitment for D3, however). Or, he may get a lot of response from D1s. That will inform you about what is reasonable for your son to do.
The best thing for a parent to do is support their kid when making choices about and once in college. I always think it is good to say “the swim team is a big commitment, it is okay if you decide that you want something else.” From the outside, we have views about what is right for our kid, but we don’t have to live it. Throughout life, there will be time-related choices to be made by your son. Do I become a PT so that I can coach my kid in little league or am I a transplant surgeon. Swimming versus pre-med decisions are not much different. In the end, he will do well whatever he chooses.