Well, if you do nothing about it NOW it will only get worse. The sooner you can stop the behavior, the dog will not develop a mentality of "hey this feels good!" and want to do it MORE!! The fact that he is also being aggressive about it is not good either. While it may be cute or innocent as a pup, when the dog is 90lbs and wants to grab a hold of everything in sight, THIS will be a problem!! Friends, children, older adults, there are dozens of human hazards there.
Working at a boarding facility Ive come across a few humpy dogs in my day. One in particular that stands out off the top of my head is a young yellow lab named "Easy" (ironic
). Anyway, we can no longer take this dog out to play with other dogs cause he grabs them the instant he sees them and humps away, he WILL NOT let go and it physically takes all of your strength to release him from a dog, and then he wants to get right back on. Not only is it a problem for other dogs, but if he chooses a dog who doesnt like to be humped it results in an instant fight. On top of that, he also has no reservations about grabbing STAFF members and has knocked down or grabbed a hold of at least half of our staff and they could not get him off without help since Easy is a big boy for a lab! Is this something you want to look forward to in the future?
Sometimes an early neutering may help ease the hormones a bit to get him to stop. But some dogs have just learned to enjoy the feeling and so they will continue to do it even years after being neutered. Thats one theory (hormones).
The other is that puppy is trying to establish his place within your pack. Sometimes dogs may try to show their dominance over people or other dogs by mounting this. You typically see this if you see a female humping a male dog, or a dog that really never humps try to hump another dog. This is their way of saying "Hey, Im tougher/stronger/more alpha than you so you submit to me." If THIS is the case in your pup, I would put a stop to it immediately since if you let it continue youre only letting puppy think that HE rules the house and everyone else around is inferior to him!!!
Since your other dog will not put him in his place, YOU guys need to take the responsibility to set rules and boundaries for him. Whether you take a spray bottle and spritz him everytime you catch him humping, or you use a "time out" method and lock him in another room for 1 minute intervals everytime you catch him. I suspect that hes a dominant one from the litter and is testing out his dominance on your family since he's new to the pack.
You could also shake a tin can full of pennies at him, screech loudly in a sharp high tone (thats puppy-puppy language for STOP!!), or even just entice him off with a piece of food and make him do some obedience commands (sit, down, rollover) before giving him the treat so he doesnt think you are rewarding his humping.
