When a new dog comes into the house trey grabs his bed and does his business usually with us saying "hey trey knock it off!" Or "aagh trey gross!" When he was really young, we got him at 10 weeks he was doing it a couple weeks later. He came into the house with 2 female dogs, and he would have these rages where he would do the.. hump walk around my brother. Who is a growing teenager. Either boyfriend has a stronger scent of male pheromones, he could have been sweating, or just smells like a dominant male and hank thinks, alright buddy this is my turf! It could be his stature, the way he talks, deep voice, anything. Just hank being hank I'm sure haha! I wouldn't worry about it but just let him know he doesn't need to do that to show hes a big boy!