You won’t change his mind by being nice to him. I know it sounds counterintuitive but trust me — think back to when you were young and had a BF that you were wavering about. What would have made you keener on him? Him being super loving, attentive, present and affectionate? Or him giving you space, being absent, giving you peace to decide how you felt?
If you’d really gone off him, neither would work! But if you were at all on the fence, then a more absent, silent BF would’ve won you round more than a clingy one.
So do the 180. Withdraw (not emotionally; just remove yourself from his presence as much as you can, fill your time in a different room, don’t draw him into your world). Create routines for yourself that don’t include him. Get legal advice as to where you will stand financially if you split. Make contingency plans.
It’d take a LOT of self-control to do this, but it’d be great if you could try it.
Chasing him for sex (“hysterical bonding”) won’t work.
Asking him to talk about his feelings don’t work. If he’s met someone else, he’ll only lie and invent reasons that will inevitably hurt your feelings. If he’s just confused, he’ll invent reasons that will be vague and nonsensical and only leave you confused.
Retreat. Give him all the space in the universe.
If he can truly lose interest in two weeks, he is not your Forever partner. He is just a bloke. And a messed-up bloke at that.