Op, it sounds like you are over worrying about your son, maybe because you have such an active social life, you expect him to have the same when home? Or maybe you want him to be seen to, by your friends?
My DS is now in 3rd yr at uni (but actually on a placement year). He was part of a mixed group of about 12 in latter school years, and made a great group of friends at uni. During first year, his school friends met up at all hols, but that dropped off in second year, I guess as Easter and Xmas are important pre-exam times when at uni.
DS had June at home prior to starting placement, think he met local friends who were around just once, but lots were either not yet back or away with uni friends. One girl has completely blanked the group in last 12 months, for whatever reason.
He has been home for a week now and not seen his group. Some went out last Sunday but PIL were here and he preferred to see them. He is meeting a different school friend for coffee today but as of yesterday, has no plans to see the main group or go out on NYE. No fallings out, it’s just not happened, assume lots will be busy with post Xmas exams and family time.
He isn’t bothered, we aren’t bothered, he is very happy generally and his life is moving on away from our home town, despite him still coming back to see us regularly. The difference is that we don’t have a busy social life, so don’t feel we have to push him to be out Xmas eve or NYE. We have all been enjoying the holiday doing family walks in daytime and chilling in front of TV in evenings.
I can see DS having less and less social interaction with local friends when home - 1 or 2 have never moved away but those that did, will start to graduate and move with jobs this year, which will also change the dynamic.
Incidently, DD18 has not gone away, nor have some of her friends and she has other friends back from uni too. As this is her first legal NYE, she is desperate to go out, but most of her friends don’t seem bothered! Maybe the younger generation don’t set as much stall at being out on NYE??