this is just puppy behavior, not aggression…and this doesn’t need discipline, but it sure does need a lot of patience! your best bet, here? offer puppy-appropriate toys that can accommodate baby teeth – any petsmart or petco will carry them. i like plush toys as well as puppy-specific Nylabones and Kongs. every time he starts chewing on something he shouldn’t, say “no” gently but firmly and give him a chew-appropriate toy. it’s just that simple! at this age, too, he shouldn’t be more than a couple of feet from you – shut doors to rooms that aren’t puppy-proofed, and keep clothing and shoes out of his reach. be consistent, remove bad things from his reach, and eventually he’ll start going for things he knows are “his” without you telling him to do so.
now, every time that puppy nips you, squawk loudly like a puppy would and remove yourself from play. this might mean you leave the room and shut a door between you and the puppy…and it should just be for a couple of minutes, long enough to get the point across but not long enough to leave puppy unattended. be consistent. he’ll figure it out eventually!
good luck! puppies are a handful. i don’t envy your position at all. 😉
this is a training issue. i would ask a trainer for help to ensure you’re doing things right, but i would keep both dogs leashed when they’re together…and crate them when they aren’t, if you live together.
Have both of these dogs been neutered? (I assume they are male). If they aren’t, get them to a vet now. Mounting is all about who is in control — not about sex. But, having all those hormones in their bodies amps up all reactions.
If you are making a fuss, and there is snapping and barking involved it sounds to me that the humans are not the ones in control in your life. Both you and your boyfriend need to start working with your dogs on the basics such as down stay, sit stay, go to your bed, bring me a toy, find the ball, heel, etc. As you gain control over the mind of your dogs you show them that humans are the ones in control and they will feel less need to show dominance to each other. It wouldn’t hurt to find a trainer to help the four of you!
Along with this work to calm your dogs with strong leadership, your best tool might be a loaded water gun, or spray bottle. No yelling. No whistle. No scolding. Just pick up the gun/bottle and spray the mounting dog in the face. When he dismounts he needs to be redirected: “find your ball”, “go to bed”, “bring me your toy” or “go outside” (this is why you need to teach these types of commands to your dog). The water needs to happen so fast, and with no fanfare — it works best if the dog never figures out that YOU are the one making the water happen. Keep the guns/bottles all over the house so you can get to one in a step or two. Don’t try to drown the mounting dog, it is just on shot to his head/face to startle him and break his focus on dominating the younger dog. It sets up the association in his mind that when he dominates the other dog, this uncomfortable and startling thing happens. The the redirection to a toy or bed channels his mind in a better activity.
Scolding, blowing whistles, and shouting escalate the excitement — you are barking along with them in the mind of a dog. Stop. Be calm. Calm, positive energy, and 15 mins of focused training 2 times per day (at least) is what is needed to improve the situation.
Good luck.