Most w/c snakes are loaded with endoparasites and that should be the first suspicion in a non-feeding snake. Even though he has done well up to this point, most snakes do tolerate a certain level of parasitism. Then certain conditions will allow those levels to rise and cause the inappetance. Of course it doesn't have to be a parasite but as soon as you see the inappetance that should be ruled out. Especially in w/c individuals.