Of course it's not 'common' for 15 extended family members to go away together. 🙄And especially not regularly. Most people just go with their partner and children.
Your family situation sounds suffocating, cliquey, and a bit toxic @LindPat All meeting all the time, doing most stuff together, having meals together a lot, and socializing together, and getting snitty and giving the side eye to family members who don't comply. I'm not surprised that your son is giving you all a wide berth, and that 2 other family members have gone to Uni. Don't be surprised if your son moves further away. YABU, and very much so.
My mother was like you (and so were several of my aunts and grandmother(s.) Would try to make you feel like shit if you didn't come and visit them 2-3 times a week and ring them at least 3 times a week in between. They'd never ring you though. (And rarely came to visit.) Oh nooooo, it was always up to the CHILD to do the running. Even when said 'child' was 30-something, or 40-something, with a busy job/career, children of their own, friends, hobbies, a social life, and a home to look after. The passive aggressive shit I got was outrageous, and I was always made to feel bad if I didn't visit my mum 2-3 times a week. (As well as ringing her 3 times a week in between.)
Consequently, I never EVER make my 2 adult DC feel bad if they go a few weeks without visiting us. I go see them every couple of weeks and they come visit me and DH about once every 4-5 weeks. We speak on WhatsApp, several times a week, and chat on facebook and twitter, and we are all happy with the level of contact. They are 17-20 miles away, (about 25-30 minutes drive,) and I know they'd be here immediately in an emergency. They know we would be there for them too.
I don't have a deep desperation for my 2 adult DC (both aged 30-ish) to constantly visit me, because I have a life of my own. A part time job, hobbies, friends, and a busy life.
I think you need to get out more and get more hobbies @LindPat You keep on at your son, and try blaming his girlfriend for his actions, and you will drive him away for good.