Thanks for you responses everyone.

Pinot's Mom's post was very helpful. Well, I guess it would be due to his renal failure. He got a shot of iron at the vet on Saturday, and I've never seen him do this before.
Of course I will try to stop him from doing this. But that's difficult when I'm at work and at night when I'm sleeping. I guess I'll have to see about changing his litter. I really don't want to do that, but if it will help then it will be done.

Thanks again!