Well, if you are unable to at least remove the cover and wash it i would suggest you get a pet stain/odor enzyme remover. There is one you can buy in the States or on the internet called "Natures Miracle" and what you do is drench the soiled area so the remover reaches down as far as the urine. You let it dry naturally and as it dries the enzymes "eat" the urine and odor cause smell. Sometimes a second drenching will be needed.\
You can also use white vinegar and drench the area on the sofa and allow it to dry naturally. You will of course have a vinegar smell for a few days until it dissipates, but the smell will fade and you can then use some fabreez or something to freshen the the fabric. The thing is you really need to get rid of the odor or kitty might think this is a new toilet for him. I'm glad you dont sound too mad at him.
Hope these help.