Its a dominance thing i think, plus a little bit of normal male dog behaviour. My Dozer did that when he was a puppy at the same age. He is now 10 months old. We spayed him when he was 7 months old, and he really calmed down a lot, he is not as hyper, and the humping has stopped. I would stop him from doing it when he did, and tell him no, but it is a temporary phase.