Cat pooping outside litterbox?

Okay, I am really lost right now.

I have 4 cats.

One of them is a cat I got from a hoarding case (used to work at a shelter, and he would have been euthanized otherwise). He's extremely inbred (the hoarder thought by breeding a bunch of cats that looked vaguely Siamese together she would end up with purebred Siamese...he's about 6 generations directly inbred, eek). He may have FIP, but since there is no specific test we are not entirely sure. If he does, it is the dry form.

He has not had a solid stool since the day he was born. Nothing helps that, and he is comfortable. He's a very happy go lucky cat, though it is obvious he does have mental problems - that sounds odd, but it's true.

We had to switch to high wall litterboxes because he stands to poop, and it sprays everywhere (ew, I know). But lately, he is pooping everywhere. On the carpet, on bedside table, coffee table, right outside the box. He doesn't appear to be stressed, though. We've added even more boxes with various types of litter - that worked for a couple weeks, but now he is back to going on the floor.

What do you recommend?

He is seeing the vet on Monday.

I really don't know what to do anymore. Do they make cat diapers?? Right now he is living in the bathroom because it's tile floor, and we rent.

There have been no major changes or stresses introduced, as far as we know his health is fine but we are double checking. We may put him back on depo-medrol injections once a month...they didn't help much, because going to the vet stresses him out BADLY, but when he was getting them he always used the box, but at that point he had always used the box before that too.

I do not want to euthanize him unless he is in pain we cannot fix, or unless he will be permanently stressed or unhappy. We definitely can't rehome him (and I wouldn't anyway) but I want to come up with a way to get him to use the box. He always pees in the box, but he just won't poop there.

Any suggestions?? Thanks in advance :)