Why has my cat stopped using his litter box?

Try leaving your cat and the litter box in one room for a day or two. See if he'll start using the litter box again. If not, there might be something wrong that is making him feel uncomfortable to go in his litter box. My cat ended up urinating along a wall and I found out from the vet she had crystals in her urine that made her extremely uncomfortable. Hopefully he's just being stubborn but give it a try.