Humping often is about being top dog, not sex. Spay/neutering usually reduces it, but dogs with more drive for dominance will continue. Some dogs with a strong drive for dominance will express it in different ways. In a wolf pack, each wolf has its rank within its gender with very little cross competition. That is why experts always suggest avoiding getting another dog as the same sex as you have. So your boy is just trying to let the other boys know he outranks them. If he hadn't been neutered, he might be more interested in the females. Of course, dogs don't read some of the stuff you have. Some neutered males will still mate with a female in heat. Never any puppies, but sometimes an infection.
You need to give him strong leadership. Life is much easier if the 2 legged pack members outrank the 4 legged ones. You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at
Raising Your Dog with the Monks of New Skete For more on being top dog, see
Establishing and Keeping Alpha Position One big mistake many people make with small dogs is to not take them seriously and do obedience training.
Thank you for giving a mixed breed a home. We have too many more nice dog than good homes.