Several things to try:

Take your cat to the vet to rule out any physical problems.

Move the cat box to a quiet area--cats like privacy.

Set up another cat box--some cats like to poop in one and pee in the other.

Keep the boxes scrupulously clean; scoop out a couple of times a day.

You may need to try different kitty litters, different depths of the litter,
different cat boxes.  (change one thing at a time so you can tell what if
anything helps).   You may need to experiment a bit to find out his
preferences.

Use an enzymatic cleaner to clean the spot where he is pooping so he doesn't
smell previous accidents.

If he's pooping in the same spot every time, try putting down crinkled foil
where he is soiling to make it less attractive to him.

All I can think of right now but I'm sure other people will have other
ideas.

Good luck, Bonnie

First, as another poster recommended, you should consult with your vet if
your cat begins displaying abnormal behavior. You just want to ensure that
there's no serious health issues underlying his behavior. Not too long ago,
my cat, Sassy, began soiling my throw rugs. Sometimes she'd go in her litter
box, sometimes not. After I took her to the vet to ensure it wasn't
something physically wrong with her, the vet recommended that I try a litter
called, "Cat Attract". It's more expensive than your average litter and is
heavy to carry, but it *really* works. At least it did for my cat. Because
it was expensive, at first, I mixed it with my normal litter 75% Cat Attract
to 25% my litter. She went right to it and never again went outside the box.
Over time -- once she had reestablished good litterbox habits, I diminished
the amount and now I don't use it at all. I'm back to my old litter 100%.
She's been fine ever since. I don't know whether it will work for you, but
it's worth a try. It did wonders for my cat.

Here's a link to the product so that you can learn more about it, but you
can buy it at many pet stores or on line, through Amazon or other places.

http://www.preciouscat.com/

Sandy