Does your dog seem to like his "special" little treats out of the litter box? While this might be the most disgusting habit your dog has, it is one you can train him out of. Eating out of the litter box is unsanitary, gives your pup poop breath, and may transmit parasites to your dog. On top of all this, your cats are not likely to be very happy to have your pup raiding their private place. They may even sneak off and find a new place in your home to eliminate, making a big mess for you to clean up.
Oddly enough, there is plenty of nutrition in your cat's waste. Nutrition that, believe it or not, is good for your dog. The only real problem is that this nutrition is trapped inside your cat's poop, making it a poor choice for a puppy snack.
Eating poop is officially known as "coprophagia" and the reason your dog likes to eat cat poop is that, to him, it tastes good. They come by this habit naturally as mother dogs will eat their puppy's feces in the den to help keep it clean. Most dogs will grow out of this habit on their own after about 9 months of age. It is up to you to put a stop to it if your dog doesn't grow out this nasty habit on his own.
You could try to block your pup out of the room with the litter box in it, but depending on where the litter box is located, this can be more effort than it is worth. However, you may want to use this method when you aren't around to watch your pup.
The problem is that your dog is very definitely attracted to this stinky snack, and is not likely to want to give it up. During the early stages of training, you will need to keep your litter box as clean as possible. Your pup will need to have mastered commands such as 'come', 'sit', 'stay', 'down', and 'leave it' before training him to stay out of the litter box.
To get started, be sure to have treats and toys on hand for training purposes. One method uses deterrents, including a food additive for your cat and a spray bottle filled with water. You'll also need plenty of time and patience for trying to teach your pup not to keep eating his favorite snack.