Look very very closely at the programs before you sign up for anything. The vast majority of them are simply housed at the university- an outside program that rents space from the university. That doesn’t mean that it isn’t a good program- but obviously it means that you want to check out the organizer’s credentials, and make sure you aren’t just paying for the zip code.
One of my D’s did a summer program at Brown, which she enjoyed thoroughly. But, she had two reasons for doing the program: the course leaders were people that she was interested in studying with, and she was considering applying to Brown (even though it is not the same in the summer as during term, it is in a region she was unfamiliar with) (follow-up: she is majoring in the subject, but chose not to apply to Brown).
Another did a summer program at Stanford, offered by Stanford directly, which was outstanding. It is a full undergraduate course, not a ‘summer program’.
Another did an “Intro to Medicine” course at Georgetown (which is when we learned the difference between outside programs and university-run ones). She actually enjoyed it quite a lot, and they did get to do a few cool things (viewing surgery, for example). She also did IR in that session, and by the end had decided not to go towards medicine, so there’s that. She got a lot of peripheral things from the program though: they took the kids on a lot of college visits during the term, and she met a fair few students that she has run into again at other things (at Model UN meets, for example). She also got experience living in a dorm, doing her laundry, having a roommate, etc. So, overall it had some merit.
It is hard for a high-schooler interested in medicine, as even volunteers are typically required to be 18 before they can be around patients.
fyi, all the courses at Oxford are run by outside groups. Also, although I am pretty sure that this is not relevant to your son, fyi the selective US universities are not impressed by any of the programs in and of themselves- the adcomms tend to see them simply as evidence of the parents ability to pay for fancy summer camp (as the Dean of Admissions at Princeton put it). (accredited summer classes at a university are a different matter).
I know students that have done the NIH summer internships: http://www.niehs.nih.gov/careers/research/summers/ and as 4beardolls notes, local professors will sometimes have work- easiest if your S has an area of interest that he can talk about a bit. D2 did that one summer in HS as a volunteer and at the end of the summer they gave her a small stipend by way of thanks.