Something similar has happend to us. I used Downy liquid fabric softener diluted with water in a spray bottle and sprayed the affected area heavily for several days. It helped get rid of the smell and it is also suppose to deter the cat from spraying there again. Once you've gotten rid of the smell spritz the area with the solution about every three days or so for a while and that should keep him from coming back.

Febreeze would even probably work and help get rid of the smell. It finally got rid of the urine smell from Disney's vent accident in the bathroom.

Hope that helps.