Keep him confined to one small room, like a bedroom or a bathroom. and make sure he knows where the litter box. PLace him in the litter box so he knows where it is. Cats are smart, and their natural instict will hopefully make him use the litterbox because he'll want someplace to dig. Just make sure you don't have a lot of clothes or junk lying around the room because he may want to dig in that. Is he neutered? that will also make litter training a lot easier.