Dogs aren't like humans. They don't get "aroused". His penis comes out of the sheeth when he's humping because he's preparing to penetrate. Even though he's young this is all instinctual. He's doing this because he for some reason thinks he has to. It could be a dominance thing, but it could also be that your female is putting off a scent that is drawing him to do it even if she's spayed. And yes, he may emit semen. I'm not sure if it's the same type of ejaculation as humans, but I know my little man did before I had him neutered.
Neutering most likely will help, but I've heard cases where it doesn't. In those cases they seem to hump everything then with no distinction, male or female. I don't think lifting his leg to urinate has anything to do with it. If he's small, it's a matter of physics. My little guy is only 2.5lbs, 3lbs on a good day and he often does not lift his leg but instead does a kind of modified squat with one leg off the ground.
Hope this helps, |