I’m the same. 34, and extremely low contact with my mother. Been with my partner for 7 years and they have never met because she’s so difficult and quite frankly I consider him my family, not her. We haven’t got married in part because I can’t bear not having my dad there, but I know my mother would destroy everything. I visit once a year for Christmas and typically come back early, in tears, with my mental health in tatters. My dad accepts she is manipulative and emotionally abusive of me but is too scared of challenging her because it then makes his life so hard.
OP, you need to let your son work through everything by himself, the move, the relationship, jobs, bills, and yes, his relationship with you. At times he might fail or have regrets, and he’ll have to work out how to pick himself up. You don’t have to enjoy the idea of failing, but that’s life, we all have things go wrong and we all have to work out what comes next. And we learn from that. You insisting on coddling him or preventing him from doing it will not equip him for the rest of his adult life. People can’t find jobs or lose jobs, struggle with working out how to make ends meet, have relationships break down, get evicted, at all ages and you cannot protect him from this his entire life. In many ways, better to learn how to fail while young!
And as someone who has a turbulent relationship with my mother, I’d advise you to stop pushing contact. Mine texts or emails every day regardless of whether I respond and I find it so stifling and controlling. As others have said, be positive about his choices even if you don’t agree with them, offer your support in making the move, say you hope you’ll catch up soon and then leave him to it and let him come to you.