Some pups start early. Max had one toy -- a sheep -- that brought this out in him at 14 weeks. I put the sheep away. He also started leg lifting to pee at this early age.
If you want to discourage the behavior, you can redirect him every time you see it. Try engaging him in play, even if it is just for a couple throws of fetch. If he humps human legs, I would definitely discourage that with a "down" or even toned "no."
That said, Teddy does occasionally hump Max when he is excited. We tell him to stop and he does.
Neutering will help, but will not necessarily end the behavior.
