Does your cat go outside or is he strictly indoors? I agree that the problem is most likely from the neighborhood cats. Spraying is territorial and stress related and your cat feels threatened. Using an odor neutralizing cleaner on all of areas he has sprayed may help because he's going to continue to spray where he has left his scent. There are also pheromone products that you can spray on the areas he's sprayed that are supposed to discourage spraying. You would also have to take the same steps to discourage the other cats from spraying. I would bet if they stopped, he'd stop. Try a google search for more information and good luck!
Stella - sweet, timid, elegant, lovely, lively, amazing
Luna - large, unattractive, naughty, adored