Be sure to clean the spot where he did spray really well, is the airfreshener working? There's a green apple product or something out there especially for pet odors you might want to try. If it still smells like a litterbox, it's probably going to be used as a litterbox again

And yes, as the other posters said, do get him neutered. The spraying is something lots of male cats will do, and it will often stop if you get him neutered in time. The older a cat is when you neuter him, the more chance the problem will continue.