Hmm. What about just putting an old towel in the bottom of the litter box, rather than litter? Would he use that? Then you still have to clean up the towel and wash it regularly, but at least he is going in one place. Or even old crumpled up newspapers like folks used to use before litter was common? If you try the newspaper, you need to be sure to keep the current day's paper in a closet or somthing so he won't be going in 2 different places.

Just thinking out loud. I've not had to deal with this.