Why does your cat still spray when he has been neutered?

My guess would be your cat was neutered relatively late in life, after he learned sexual behaviors such as spraying or caterwauling. If a tom cat is neutered prior to learning these behaviors, they tend to not express them. However, once a tom cat has learned these behaviors, even in the absence of testosterone from testicles they will still express the behaviors.

It is also possible that a small portion of testicle was left during the surgery, although this is extremely rare.