Hard to know what to do for the best.
Homesickness is awful. I boarded from age 10 and back then you weren't even allowed to phone your parents for the first 3 weeks, only write letters. You were also given no spare seconds to mope. Weekends and evenings were full of compulsory activities. Now I'm not suggesting for a moment that you do anything like this. It's bordering on barbaric. But, that way of helping much younger children cope with their homesickness developed because it was effective with most children, most of the time.
My autistic son is younger but I think I would approach it from almost a psychology point of view. DS, this adjustment and homesickness stage is your brain's normal reaction to this change & stress. It's ok for it to do that, it's doing its job of and it will recede as you get used to your new environment. You can help that happen quicker by putting a good routine in place, learning your way round etc. Approach it like anxiety - get him to notice the physical signs of stress and the thoughts that are running away, then take deep breaths to address the fast heartrate and challenge the assumptions behind those spiralling thoughts. Break the cycle. Then scaffold, scaffold, scaffold to help him structure his days to keep himself busy. Get him to set himself tasks to accomplish each day - anything from talk to a housemate, catch a bus, talk to a lecturer, visit a shop and buy milk, buy a chocolate bar from the SU shop. Schedule phone calls and texts with him. Can he phone grandparents on Saturdays, and a mate from school on Tuesdays? Does he have a plan for when he'll buy food, has he found a cashpoint (might be redundant now!), has he found his lecturer theatres, registered with GP and dentist?
BUT if you can, do it all over the phone. If you spend time talking on the phone and that supports him being able to get out there in his new life (without your presence) then it's a positive thing that is building towards him taking over the reins. It's ok if that life is "smaller" than the one he might have imagined. Once he gets into his learning and meets his coursemates he'll find plenty to occupy himself with, but until then maybe some hands-off support with creating a routine and some reassurance that homesickness is an expected stage but not the "new normal", might help.
Yes, a small campus uni might have been a bit easier but the immediate problem, I think, is getting him to feel comfortable in his new room/flat/hall. Once that starts to feel like a safer place to him, he will have somewhere less stressful to go and he will be able to cope with other challenges. That would be the same problem whichever uni he is at, and it CAN usually be overcome.