You can teach him a command like “leave it” or “go to your bed” and use it whenever he goes near the litter box.
Keep doing this regularly and give him treats or praise when he listens. Eventually, he’ll understand that the command means he should stay away from the litter box.
When I encountered the same issue before, I had to move the litter box to a place the dogs could not get to. I had no option but to place the litter box in the laundry room and install a kitty door. When the dogs got too rowdy, one of the cats enjoyed having a secure area to retreat to.
In a separate incident, a friend of mine placed the litterbox inside a medicine cabinet. The door to the nearby cabinet was taken out. The dog was deterred from entering the room by the cat’s ability to navigate beneath the sink’s pipes and reach the litter box.