I echo what Debbie and cybercat said:

1. How often do you scoop the litter box? Minimum, it should be
scooped daily or twice a day if possible. Also, periodically, all of
the litter should be replaced and the box cleaned.
2. How many boxes do you have? Try adding one or two boxes in new
locations. Some cats like to urinate in one box and defecate in
another.
3. Have you changed brands of litter recently?
4. Do you use a scented litter? Some cats (and humans like me!) hate
perfumed litter. I buy only unscented.
5. Take him to the vet. There might be a medical problem.