I've never heard of that happening as a result of neutering. It takes a few weeks for the neutering to affect their systems and reduce the testosterone levels. Is it possible that there is a female dog in heat nearby that he can smell? Or is there a new animal or person in the house? Humping is often a show of dominance. Some neutered males and even females will display it. If he was already displaying this behavior before 6 months of age, then he may just be predisposed to it. Neutering usually helps reduce the behavior, but the affect is not immediate.