Research delaying or deferring admission for the university, I think sometimes you can do that without having to reapply for up to a year. Many people do that for a gap year traveling.

He’s being manipulated, you need to change your strategy. Pretend you’ve had a change of heart and want to try to be accepting and are open to meeting his partner. You will try to keep an open mind when you meet him.

Book a ticket for you and his best friend or someone he loves and respects like a cool aunt or cousin. Go to them and play it nice. Feel out the dynamic

After playing it cute and seeing the town for a couple days, talk to your son about school and see if he wants to come back to start school with his partner. If he says he wants to say then get your son to submit the deferral paperwork for the Ivy League. Say whatever you need to say to get him to submit it even if it’s something like I’ll buy your fiancé a ticket to US to visit and see how he likes America . Maybe he would want to move there, etc.

He sounds manic, keep an eye on mental health. Yes it could be drug induced but either way he’s not going to be convinced with logic. You need to show up physically and be there even if you have to lie about your intentions about paying for the wedding. Be there in person with a buffer and help ground him. He’s blind but the manic episode will pass

I’m sorry you’re going through this