Loads of students feel exactly the same as your DS when they start university. It's a massive change and it is difficult, especially if you're shy. The first weeks are by far the hardest and it can take a while to settle in, especially when the only people you're meeting are the ones in your accommodation who you might not necessarily have much in common with.
I hated my first university and I did end up dropping out after one term. It wasn't the right university for me, it wasn't very campus-based, the people near me in halls were very cliquey, and I then had another issue in my life that made it all too much. However, I reapplied for different universities the following year and had a completely different experience - yes, it was still terrifying and yes, Fresher's Week etc was really stressful, but I was totally happy and settled there by Christmas.
I would suggest that you tell him he needs to stick it out for a term, ideally two. And he does absolutely need to make more effort with his flatmates instead not joining in because he 'feels like he'd be in the way' - they probably think he doesn't like them and doesn't want to participate. I think it can be hard if you're not interested in partying, because a lot of Fresher's Week activities are about that, but I think you have to force yourself to go along with it up to a point purely to meet people (you don't have to get hammered, just go along and nurse the same drink for hours if need be) and get used to the social scene a bit. Joining clubs etc is also a good idea. They are very good for meeting like-minded people. He'll also meet more people through his course when that properly gets going, and he's likely to have more in common with them. His department will almost certainly have a society for social events based around the subject too, eg there's usually a Maths Soc and and English Soc and so on.
Also - has your DS ever had a job? I only ask because I massively, massively grew in confidence after I worked in a shop for year between universities. I'd had a Saturday job before that but working full time in a team and having regular customers and more responsibility etc just made a huge difference to my shyness. So if the worst comes to the worst and your DS really doesn't settle in his first year, i'd definitely recommend that before he tries again at uni.