Cat urinating on a couch

Question
My partner and i--both in our 60s--recently adopted our 10th cats.  The others have all passed away. The two we now have, supplied by the anti-cruelty soeicty are siblings, brother and sister from the same litter, who have been together their entire nine months of life.
Now, the male cat Nathan, has been urinating on a crack between pillows on an expensive mohair couch.  I suspected he might be doing it four weeks ago when i saw him crouching and it appeared about to urinate/.  i interrupted him and he headed for the litter box, which he uses regularly-=-but not regularly enough i guess.

Today Barbara saw him wet the couch and checked for more evidence, found some stains down in the crack of the couch.  now we wonder what to do to protect our couch and change Nathan's behavior.

Any ideas?

Answer
Hello Hank!

I presume Hank is neutered since you got him from a rescue group.  If not......be sure and have him neutered as sometimes , that is the problem.
Cats will sometimes urinate out of their litter box when they are having urinary tract problems.  Male cats are notorious for doing this.  If you see him straining... or going to the litter box (or couch) frequently.....then take him to the vet as soon as you can.  Males will get stopped up and it is life threatening.   I'm not trying to scare you, but you need to be sure and rule this out before you go any further.  I had a male do this, and lucky for me he was going in the sink, so it wasn't a problem like you have.   
Be sure to buy a good pet odor cleaner or shampoo and clean your couch thoroughly so that he won't keep smelling the urine and wanting to use it again.  If it were me... I would confine him to a room with the litter box , when you're gone,  until this habit stops.   If you don't,  I'm afraid he will continue to use the couch.  
A friend of mine had a similar problem, and his cat was using the carpet in his hallway.  After cleaning it good, and confining his cat for a week or so,  he began to slowly let him out.  He also put aluminum foil over the area where he went, as cats do NOT like it.   Eventually, his cat was "retrained" and began using the litter box normally.  I hope this helps you too.
But first,  please make sure it is not a medical condition causing him to urinate there.

God Bless,
Becky