Although Ds liked his course his first year flat was very divided in terms of a hell of a mix of people. When I went to uni there was no entertainment in your room, you had to physically attend lectures, study in the library, typing up work in the computer lab, meeting up with people was easier because you saw more people out and about. I am introvert and this really helped me socialise.
Today they have online lectures, they have laptops and internet connection for doing work and sourcing information. They have Netflix, gaming consoles, Youtube etc. It seems easier for introverts and anyone else to just stay inside their rooms. It could well be that he feels the same at another university. Or he hates the people on the athletics team.
Re the flat, is it university accommodation? Has he looked into moving or swapping? Seen the accommodation officer over it? As for second year accommodation are there private halls he could go into as a single? Lots of people end up with a single room to fill too.
Have you personally looked at all the clubs and societies that the uni offers? I think uni is sold as this amazing time where you meet friends for life and have the best time and for a lot of students this isn't the case, it is just okay and learning to just get on with things can be a good thing.
In all honesty he actually hasn't got long to go to complete first year, they have this semester, a teeny bit of the next one, exams and then home. Ds finished 4 weeks before the end of term in first year.
The biggest and most important thing in all of this is his grades. Even with entry grades in hand it is no guarantee he would get accepted onto a course. It is the minimum they will accept applicants in on and in your son's case he was lucky to be accepted with a grade drop.
Have you both looked at the unis with BBB entry grades for his course and would he want to apply to any of them? What is his plan B if he drops out this year and fails to get accepted in another uni? A lot of unis do hold out to see if those with higher predicted grades apply, ie 3 A stars across the board. At least one uni openly admits their entry grade is AAA but they tier their offers; their first offers go to 3 A stars or above, 2 A stars A, 1 A star AA, then AAA sometimes all the places have gone before they get to AAA.