I lived in the dorms for two years as an undergraduate, and hated every second of it. I'd have given anything to commute from home (but given that home was a 5 hour drive away, this would have been impractical). I also know people who absolutely loved living in the dorms. The OP and son have made a decision that works for them, and these "friends" need to butt out.
I think that the best solution, since they are apparently extremely persistent, is to tell them, firmly and politely, that everyone involved is happy with the living arrangements, that they are not up for discussion, and ask that they not bring the subject up again, since it's intrusive. Hopefully they'll get the message.