Cats not using litter boxes either have a behavioral or medical condition. When it is a medical case it could include a common disease like feline lower urinary tract infection. Signs of this are urinating frequently in small amounts or it can be very painful while urinating to them.
Stress is usually the primary issue for cats not using their box. This would be a behavioral case. This stress could be caused by many different things such as a change in routine or even having a visitor at your house.
Make sure to clean out the litter box on a regular basis. That way it has a nice smell to your cat, he is very clean and doesn't like an unclean toilet, would you. This could help your problem. If not, take him to the vet to be checked to make sure everything is alright. Your cat could have problems, which he might associate the litter box with the pain he is having and not want to use the box.
Another thing is where to place it; it can have an effect on your cat. Cats are very shy toilters! They like privacy while using their box and make sure it is in a quiet area and not around the busy part of your house. You should have one box per cat, plus one.
If you think your cat has health issues of why he is not using the litter box, it is best to take him to the vet to get him checked out and go from there.
On more cat related topics you can go to:
Help your cat avoid urinary tract infections