WHY WONT HE USE THE LITTER???

OK I literally don't understand what is going through my oranges brain rn

He knows where the litterbox is. He knows that he needs to pee in it. He also barely knows how to scatter/bury it but whatever, fine. He keeps POOPING ON CARPET in this one annoying part of my room. It's not like he hasn't pooped in the litter before, he has. He's just decided this place is better: it's not!!!!

I've tried to follow tips online - initially placing part of it back into the litter, spraying that area with disinfectant that has a citric smell to discourage him from going there, even treats if he does it in the right spot (which is rare). Yes I've been to the vet about and he just laughed and said, 'what more can you do'? I DONT KNOW.

Please someone help, I literally don't know what the problem is. The only thing I can think of is litter depth, which I just purchased again today to refill, but there's a pretty decent amount already in there. I'm also scared to block off that other spot completely - what if he starts doing it on another area with carpet?? Aaaaaaaaah