Do you pick up the poop and put it in the box? I would pick one litter type and stick with it, but agree that a vet check is likely in order. That he pees in the box means he is okay with that litter, and getting in and out of the box, so it may be something more medical than behavioral.

I the mean time, I would surround the box with easy-to wash towels or wash clothes, just for easy clean-up and odor control.