I'm sorry to hear this. It could either be behavioral or medical. I'd take him to the vet so they can take a urine sample to rule out a medical problem. Are all of your cats spayed and neutered? If not then you need to get them fixed because this can also cause them to start spraying.

You need to also start cleaning up the areas he's sprayed with some type of enzyme cleaner or he'll continue to spray because he'll still be able to smell the urine. Have you tried using Feliway/Comfort Zone plugins or spray? This will help to calm the cats down and to also keep Sylvester from spraying. You can also try cat attract litter by Dr. Elsey which is sold at Petsmart or online. It also comes with a Litter Box Solutions Booklet that will give you a lot of great advice. I hope that something will work for you. I would hate for you to have to put Sylvester down for spraying.

As a last resort you can also ask your vet about giving him some medications to help calm him down. Kitty prozac is usually one of the best to try and some people have also had great success with clomicalm which is usually used in dogs. Good luck.