Where is he spraying? Neutered cats often spray if they spend time outdoors, they will do it at the edge of their territory - at the end of the garden for example. If it is happening indoors in the core of their territory, it is most likely due to a urinary tract infection or stress (overcrowding for indoor cats, or neighbourhood tom coming into the garden or spraying on the wall of your house or staring at your cat through a window), your first action should be to take the cat to a vet to rule out health problems, if there are no medical reasons then try Feliway
. Hope you can get it sorted out.