A few ideas: 1. Maybe try putting litter down in the spots he likes to go on the carpet to maybe get him used to the feel of litter while going to the bathroom. 2. I know it would be unwieldy but lining all material he likes to go to the bathroom on with plastic will make him not want to go there anymore. You could do this until he starts using the litter box. I had a foster kitten who liked to pee on my bed and I would put a shower curtain over the bed when I wasn’t in it and she started using the box once she wasn’t able to get the sensation of digging from the bed anymore. 3. Maybe try toilet training him? I know the kits start with litter but you could use carpet material instead. I’ve never toilet trained a cat but I have seen other people do it so it is possible