Pet Sitter Needed

Prowl is around 9 years old and is very skittish around new folk. He doesn't hiss or scratch, more likely to stay hidden. He's an indoor/outdoor, but we want him to stay inside because our new roommate is allergic to fleas, which we're still trying to combat. Problem is, he refuses to use a box. He's constantly using either our bed or our couch to pee, and an unused side room or the bathtub to poo. I've tried clay and wood litter, as well as pellets and pads, but he never keeps to the box. He used to be well behaved and perfectly trained, but somewhere along the line he stopped, and now we have to let him outside to protect our items. I don't know what else to do.