Do you think maybe you're projecting your idea of what 'good experience at uni' looks like onto him?
You seem to think that working hard for academic success is a sacrifice and not enjoyable. You seem to think the other end of that is not being as academically successful but doing 'fun' things instead like drinking, going out, seeing friends a lot, travelling, multiple girlfriends.
Perhaps your sadness is misplaced. He's not you. Not everyone enjoys the same stuff. Do you know he would even have wanted to have a wild party animal experience? Some people really do value quiet, and alone time, and learning something they're really interested in. For some people that brings them the same level of happiness that perhaps going to the pub with a group of mates brings you.
He has a solid healthy relationship, which is a good thing. No I really wouldn't be sad about someone staying with the first person they had a relationship with, as long as it was a good relationship. For a lot of people, this would be the ideal scenario.
I feel perhaps you need to try to see him as separate to you, with his own path to follow. I think it's really common for people who are more extroverted to look at their children staying in reading a book or whatever and feeling real pity for them, but it's often misplaced and unnecessary. Maybe he feels pity for those who are so disinterested in their degrees that they don't find the actual studying enjoyable and instead would much prefer to be out partying.
It's great you want to support him, but make sure you're supporting him with the problems he has and not the problems you'd have if you were living his exact life.