Many diseases or infections will cause excessive urination. If there are no medical problems, then the urination is likely a behavioral problem. Sudden changes or stress such as introducing a new cat or dog to the household, moving, or the addition of a new family member can cause a cat to stop using his litterbox, as well. If your cat is not neutered, then it is highly recommend that that be done. The litterbox should be cleaned at least once daily.
Cats can be very particular about the kind of litter they use.
If you haven't changed litter, try adding another box with a completely different type of litter in it. Stay away from litters with lots of fragrance. Place them right at or near the exact spots where he's urinating inappropriately, and scoop all boxes daily. This is just one example of things you can try. If your cat is still urinating outside the box, consult with your veterinarian or a pet behaviorist.