There are several reasons why your kitty might have stopped using the litter box. One is his litter is too dirty. This is the most common reason. Clean out his litter more often and see if this helps. Another reason could be stress. Something like moving, new furniture, a new member of the family or new animal can cause your cat to stress out and quit using the litter box. Reason three could be he is attracted to a previously soiled location. If your cat keeps returning to one spot, it may be a spot where another pet previously soiled. Make this spot less attractive to your pet if this is the reason. Have you recently changed his litter? This might cause your cat to stop using the litter box. Hope this helped and if you have any more questions, ask! 
