Watch out for poopy kisses. eeeewww

You could get a litter box with a lid and a flap. If the flap doesn't deter him or the cat is freaked out by it, take the flap off and turn the box towards the wall where the cat can get to the opening, but the dog can't. There are also these new litter boxes that look like rubber maid containers (at Petsmart) with a hole on the top so the cat has to jump down into it, but the dog can't get in it. Maybe you can move the litter box into a room that the dog never goes into. Just be sure to show the cat where you move the box so it doesn't get confused.

That's about all the advice I have for now. Good luck.