YJ - my first guess was UTI, because that will cause them to go in various places (i.e. they associate the pain with where they pee, so they move around to avoid pain when peeing, or so we have been told). We had a cat that had this problem and switched her food to a food that was supposed to help with UTI, but might call for a vet visit.
What other changes have happened in YJ's house? We have had a cat that was *very* sensitive to changes in routine in the household and would get all bitchy and pee in odd places whenever anything in the house routine changed -- He was a *sharpshooter* too, little fucker. Wrote his initials on the wall, I swear.
The other cats are definitely probably part of the problem - we have one cat right now that is a rescue - always been a little skitish - and whenever she comes out, two other cats (brothers) love to torment her - they'll linger, call her "Bitch" and "slut" and smack her on the ass until she finally hides up inside the recliner, where, feeling safe, she will hide for days (and shit and pee). WE finally locked her in a bathroom for about two months until it finally got nice enough to throw her ass outside and hike her kitty skirt up on the corner. Bitch.
Seriously, though, catnip, kitty valium? Something to calm his nerves? I'm sure the other cats beating up on him is part of the problem, and if it isn't a UTI, it's "fuck you!! I'm gonna pee on your shit!" behavior. Been through it, and it sucks. If you can't keep the other cats off him, you might have to live with the quarantine option, or find a new home. No...I don't' want you cat.