Maybe he is sick.  You should take him to the vet to rule out any
medical problems.  Sometimes if a cat is straining or having pain
while using the litter box, he'll associate the discomfort with being
in the litterbox and choose a new place to go.

Other reasons he could have quit using his litterbox:

1.  Unsanitary litterbox, smells bad, etc.  Make sure it is scooped
out at LEAST once a day, if not more often.

2.  Changes in the litter itself.  Did you get a new brand, or did
your current brand change the litter?  Tidy cats recently changed
their formula for premium litters.  Cats can tell when such a change
has happened and if they don't like a change, they'll go somewhere
else.

3.  Does the litterbox have a cover? If so, is there a chance that he
is uncomfortable when trying to stand inside the cover?  This could
lead back to the issue of a medical problem or the cover could simply
be too small for his size.  This is particularly an issue with larger
cats.

4.  Think about the location of the litterbox.  If there are a lot of
people in the area of his litterbox and he wants privacy, or there is
a lot of noise, he will probably want to go somewhere more quiet
and/or solitary.  Try sticking a second litterbox in the place where
he has chosen to go.

5.  There's also the issue of emotional reasons.  ANY kind of stress
can make a cat change his preferred litter location.  Were there any
changes to your household such as a new roommate, baby, etc?  Perhaps
he is frustrated because he can't go out as much as he used to.  Also
keep in mind the possibility of a new cat outside that your cat can
possibly see or smell, and he might be marking his territory.  

Tell your cats about this website: http://www.catnabbit.com