There can be many reasons, including dominance and even play behavior. After neutering, hormonal levels don't immediately change. The reduction in testosterone is gradual, as is the reduction of behaviors.
For sure you need to be sure he's healthy. Various medical problems, including urinary tract infections, urinary incontinence, priapism and skin allergies, can influence a cat's mounting behavior. These issues can be serious if not properly treated and require medical attention rather than behavioral treatment. A cat suffering from one of these or other medical issues often spend a lot of time licking himself, making unsuccessful trips to the litter box, crying during urination. If you notice your cat exhibiting any signs, take him to a veterinarian to rule out medical concerns.