Stinky couch

Hello everyone! I was looking for answers on this matter on the internet and found this website, so i thought I'd give it a try.

I know this is the most common question asked, but this case differs a little. Anyway I have two 1 yr old cats. (brother & sister) We've had them from kittens. No problems at all! Until .... well what happened is we went out of town for 4-5 days, and my sister was to come over and feed and do the litter. (the litter was forgotten about>!?!?!@?) ... so when we got home... there was poop on the couch and it smelt like pee. So we threw out the cusions and got new ones, and cleaned the litter.
Now over a month later the male cat, Dakota, still seems to think he can poop on the couch. He never does it in front of me, so I never catch him. His litter is cleaned every day. His sister Kaos, uses the same litter and we have no problems at all with her. I also suspect that he as well is peeing on it.

I don't want to have to keep cleaning the cushions, and I definetly don't want to have to buy a new couch (maybe i have too?) ... and I don't want to get rid of him. Why is he doing this still? How can I stop it?

I would have him checked out with the vet to eliminate any possible physical reasons first. Changes in litter box behavoir often indicate urinary or other problems and should definately be explored.

After ruling out any medical issues then I would firstly get a second litter box and try placing it around the area of the couch. If he then uses it you can gradually move it a way to a more appropriate area. I would also cover the couch with something like a shower curtain when you are not there to directly supervise.

I agree with everything TerriM wrote. Excellent advice.

I think a vet checkup is a great first step - definitely rule out medical issues FIRST.

after that, I'd restrict both cats to one room (not the room with the couch) with 2 litter boxes. one for each cat. I'd use a bathroom, laundry room, enclosed porch, etc... obviously, make sure no towels or laundry is laying around for them to accidently "go" on. once you have evidence of them using the boxes once again, slowly move them back to where they were. basically treat them as you would a kitten who is not litter box trained.

I have a sunporch which I used as my "training room" - I started my cats in there w/ the litter boxes, slowly moving them closer to the door, then inside the door, then down the hall, then finally to the laundry room.

I have no idea of the layout of your home, so i'm not sure if this is even an option, but if it is, it may be worth trying.

as for the couch - I'd line the tops of the cushions with all things that cats hate... sheets of aluminum foil, double sided tape, cut up oranges and lemons (strong citrus smells), etc... basically, I'd be sure to DETER them from stepping foot on that couch.