Opokki is right,
You have to move the litter box, as well as being gross, the actual cat litter can cause mild gastrointestinal upset if eaten.
Another option is to get to the poop before the dog, sprinke chilli powder over the top and he might think twice about eating it again.
Most dogs usually grow out of coprophagia (eating poo) but in the meantime make sure that he is eating enough and also treat for worms as they sap a dogs strength, requiring them to eat more.
To dogs, poo is just a secondary meal!
Good luck!