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.
IMO, the first thing you should do is get him to a vet ASAP. Neutered males especially are prone to blocking up completely & can die from that. Have the vet specifically test for a UTI. If that is ruled out, then look at behavioral aspects.