My cat keeps pooping the bed (and outside the litterbox)

Hello there, so.

The title is self explanatory

I adopted this cat about a month and a half ago, and since the very beggining ive been having trouble with him pooping on beds.

I thought maybe the litterbox was too small so i bought another one, roomie says it's still small. If I buy one that's really big, would that suffice for the cat?

He's using the same litter he was using in his previous household.

I've cleaned the place through and we started covering the beds while we are gone but we cant leave the cat alone because he gets too anxious.

It's not medical. I took him to the vet and even did blood tests, everything is fine. He's not constipated. He sits weird tho, like in pain, according to Google. But he doesn't have spine issues either, it seems.

I changed his litterbox to the bahtroom (the apartment is really small) and its kind of tight, but i thought he was getting better because he pooped outside the litterbox but in the bathroom.

Tonight, just now, he pooped on my roomie's bed.

He told me that if the cat does it again im gonna have to find him a new home, I don't want that. :(

What can I do?

I bought him calimg pheromones, catnip, toys.

At this point im starting to think it's because he wants attention? I'm not sure anymore, but I need a solution that actually works because this cat is everything I have in my life and I don't want him to go...

Do I buy him even a bigger litterbox? Do I make him have two even if the apartment is really small? I'm desperate :(