First, is he fixed? Most often, un-neutered male cats spray to mark their territory. Second, Is this a health issue or behavioral issue? Are you SURE its not health? Most often when a cat sorays, its because they have a serious urinary tract infection, or in males they have crystals formed that makes it hard for them to pee. They associate the pain of peeing with their box and spray outside the box.

If its behavioral - is he stressed for any reason? Are the cats fighting? Did you have any changes withing the home lately? If he's spraying over stress, the Feliway might work for you.

It could also be that he is a lot fussier about the cleanlinesso fhis box than the others. Rule of thumb is this: One box per cat, plus one. So if you have three cats, you'll need four boxes. Five cats would need 6 boxes. Spread the boxes in various locations throughout your home... one in the bathroom, one in a closet, one in a bedroom, etc. You'd be surprised where you can hide a litterbox. I have 4 cats, 4 boxes (but I am also home fulltime and can scoop as soon as something hits the litter, so its not as important as a home where the humans are out 9 or 10 hours a day.)