This.
OP, I think when he met his current partner (and their family) it’s far more likely that he saw a new way to live, with new independence, and has been focusing on building and learning from that time. When he has came to you you’ve seemed quite overbearing and have made it about how sad or worried it all makes you. The extreme negativity is telling him, essentially, that you don’t believe in him. You don’t think he’ll make it. This will make it very difficult for him to communicate with you at all, OP. Even if he genuinely wants to.
This absolutely explains his going low contact and why his partner, whom you have never met, might have the idea that you are narcissistic or manipulative. It’s certainly not came from her own interactions. I think his choosing not to introduce you, regardless of his partners gender, has been a way to protect his new blossoming and hopeful relationship from the negativity and judgement you’ve already assigned to it. The way you talk of her is very telling. But her parents aren’t suspicious of your son’s intentions, they are supporting them and allowing them to find their way together safely under their roof before spreading their wings fully. This is a huge bonus and very clever of them. Trialling cohabitation before leaping into moving to a new city and learning they maybe aren’t compatible.
I would be thanking them, OP.
Insisting he is just “following her” is degrading and infantilising him. This is his time to choose himself, to make his own mistakes and his own successes. He’s clearly trying to break away from being “mummy’s boy” and the more you respond to his need for space with smothering the further away he is likely to step.
“You can come home to mummy when the whole thing blows up” is not the way to show him you care. It’s a way to ensure he will never return, regardless of how this specific relationship works out. Jeez. Apologise and wish him the very best. Be supportive of them BOTH. Tell him you can’t wait to visit when they are settled in their first ever home together, and how exciting that prospect must be for him. That he’s a very good man to support his partner furthering her education. You’re happy he found love, you can’t wait to see the beautiful life they build together. I’d bet he’s longing to hear you just be happy for him and allowing him to be a real adult.
I think you would truly find happiness in that too, even if it seems scary.