After they are neutered, the behavior does not go away immediatly especially if they were an adult cat at the time of fixing. Wait a couple of months to see if the condition has improved.
Humping behavior is a dominance issue. He's trying to assert himself as the alpha male. Also, it takes 3-6 months after neutering before behavior changes.
If a cat is neutered after sexual maturity some behavior traits will stay and some take a while to dissipate. Stop him, use the NO that gets attention, be patient.
