Some cats just don't like to share the litter box. I would get a new litter box and new litter. Hope this helps!

FIsrt off, I'd make sure he is okay medically because many cats quit using the box when they are ill. Then, I'd say unless it's one of those Litter Maid, $125 boxes, get a new one and try putting it in a different spot, too. You may even want to place another box somewhere else to give him options, just make sure they are in a quiet place, with privacy. Don't clean the boxes with a strong disinfectant because cats don't like the smell of those. And never use ammonia as it smells a bit like urine to cats and they will try to cover it up with their own scent. Take the cat to the box frequently. If you catch the cat going somewhere you don't think is okay, Try correcting the cat from a distance, like with a spray gun or a soda can with a couple of cents in it. Just don't let the cat know YOU are correcting it. It shouldn't seem like you are punishing the cat. You may need to go back to square one with his training and place him in a small room with one box and increase his space as he becomes used to using a box again. If you need to keep him away from those inappropriate areas, try laying sheets of aluminum foil there because cats HATE foil.

you probably cant, maybe you should get him his own litter box