To put it bluntly, are you buying an education for him so that he can have a career and a livelihood? Or is your primary concern his social adjustment for the next four years, as a kind of social finishing school for an awkward kid, and you’ll worry about his obtaining the credentials to be able to support himself afterwards? Or maybe he is in the fortunate position as to have already been provided for, financially, in life, and he does not have to think about how he will support himself and maybe a family.
If obtaining the means of supporting himself is the primary goal, then I would say that is the first consideration. A kid who wants, say, mechanical engineering might have a very different list than a kid who wants to obtain a teaching certification.
That’s why I say look at his educational goals first. I attended a medium large school, with about 10K undergrads and 15K grads, but because I was in a small department, I felt that I was in a much smaller environment - I knew everyone in the dep’t, undergrads and grads. So it kind of had the best of both worlds. I think that a lot of people find their smaller group through their academic and non-academic interests. But if a person chooses a very small school, especially one that has many sports teams, many of the students then find their social life through their sports teams, which often hold social events with other sports teams. One of my friends’ kids is currently at a small LAC, wound up there as a recruited athlete. They get virtually no playing time, but are expected to be at every practice, every game, and have to travel to every away game (which takes up a lot of time), and unfortunately, there’s even nasty mean girl drama going on, on the team! But when I ask why the kid doesn’t just drop the sport, and instead focus on academics and obtaining a degree that might actually earn them a living, the reply is, “But social life on campus revolves around the sports teams, so if they drop the sport, they would be on the outs, socially.” Frats and sororities might be an option, but not everyone wants Greek life, which, frankly, is usually centered around partying, much as they tout their service activities.
For an average kid who’s somewhat introverted, I’d say that your best option is to try to figure out what he might want to study. If he has no idea what he wants to study, then a large school with many majors is more likely to give him the opportunity to try out lots of different intro classes, to try to find what he likes. He can find his group there through clubs and activities, maybe religious organizations if you guys have a religious identification. If he has an idea what he wants to study, your first step is to find schools that will match his academic interests. Size, distance, culture, “vibe” are all considerations after that, but I wouldn’t start out with that. Unless there is no academic goal other than obtaining any 4 yr degree, start with selecting schools based upon them meeting his educational and career requirements, rather than his social requirements.