Hard one. I feel for you OP. I’m hesitant to advise as there’s no one size fits all, and I wouldn’t want to suggest something that makes things worse.
However I gave some personal experience of this so this is what I would do based on what you’ve said about your son.
I would gently ignore his talk of dropping out and deferring - don’t get into a discussion about it, say he should reevaluate in April/May after trying the following few things.
Take him to the GP to discuss his health so there a record if you need it, Anti-depressants may be a good thing to try? It may give him the boost he needs to help himself.
I’d then get him back as soon as possible so he’s in his university routine. Him perking up at your suggestion to do that before is positive. The “crash” will be because he had to face the music about not having done work. But he’s done that now, tell him the hard part is over and praise him for facing his fears and being honest. Praise him fir his engagement this week.
Get him going back and accessing the university support and see how he gets on. It’s easy to think you don’t like your friends & flatmates when you don’t see them. He may find he actually doesn’t, or he may find he was dwelling on the negatives about them as a misaligned coping strategy.
After all this, in April/May he can re-evaluate whether he wants to drop out. You’ll have the evidence of the doctor’s meeting and his attempts at trying to re-engage.
However if he continues to slide then of course you may need to intervene more dramatically.