If your cat isn't using his litter box, you may need to do a little investigating. There may be a good reason why.

[edit] Steps

Look at the cat's litter. Is it changed every day? Is it scented? Try different types of litter and see if that resolves the problem.
Location of the litter box? Is it in a high traffic area or more secluded? You can even put a small folding screen around it for privacy.
How many cats are using the litter box? You should have a litter box for every 2-3 cats - they don't like to share very well.
Has anyone (person/other pet) bothered or hurt them when using the box? Sometimes cats will avoid an area or situation that caused them pain.
If this does not improve the situation, take your cat to a vet right away. There might be a more serious problem that prevents the cat from using the litter box; your vet will be able to offer professional advice.
If nothing medically wrong, then confine the cat to one room with the litter box and retrain.

[edit] Tips

Keep the litter pan clean - scoop once or twice a day.
Put the pan in a secluded/private area.
Check the type of litter used.
Switch to a covered pan or open pan - depending on which one you currently have.

Keep in mind that bad behavior at the litter box might be an indication of a medical condition. For example, your cat might be suffering from feline lower urinary tract disease (FLUTD). Not surprisingly, a special cat litter has been created that detects if your cat has Feline Lower Urinary Tract Disease. In any event, keep in mind that bad behavior might indicate a medical condition. Consult your vet when in doubt.