I think I'd have to agree with the "back to confinement" for a while. Glad he's being vet checked to rule out UTI, and hope it's that (well don't, but the alternative..) and not territorial behaviour starting up. I wonder if he was neutered much later than six months, which would explain his sacks are on the large side still. Was Tubby neutered very young? If a cat's had a chance to develop Batonga Wongas back there, they tend to stay large. Hope you solve this latest dilemma.