You must use an enzyme cleaner, that is the ONLY thing that will take care of it. You can try one from a pet store - the most popular available one out East here is Nature's Miracle. It works OK. Usually (for us) took two or three applications. We switched to this one: http://www.nokout.com/odorelim/pets/
Though we hear fabulous things about this one: http://www.catfaeries.com/cleanup.html
What you do is apply the cleaner - and a lot of it. Especially on a couch, it has to soak down into anywhere the pee might have gone. I mean really, thoroughly soaked. Let it sit for 10 - 15 minutes, then blot up as much of the damp as you can. Then it just has to air dry. Our couch would take 2 or 3 days depending upon how humid it was. What we did was cover it with aluminum foil and put towels down over it so we could sit on it. We'd pick up the towels when we weren't sitting on it so there was no "attractive" surface for continued peeing.
You may also want to consider getting a black light to make sure there aren't any more places he went that you don't know about. After dark, turn off the lights, and go through your house/apartment with the black light. Any "pee" spots will turn up as an orange splotch. http://blacklightshop.com/bl6.html
Is he neutered? If not, if he's older than four or five months old, he should be. That will stop behavior like that. If he is neutered, keep an eye on him or just get him to the vet. Peeing outside the box is usually a sign of a medical problem.