We had a similar problem with our dog. We adopted him at about the same age.
We found that putting our hands around his muzzle, looking him in the eye, and scolding him pretty much solved the problem.
When you put your hands around the muzzle, you have your hands on top of the nose. This extablishes your dominance. Looking him in the eye also stresses your dominance. Holding his mouth closed specifically corrects the “bad” behavior.
Maybe this will wori for you, too.
Okay, here is a suggestion for correcting a young dog/puppy who is behaving inappropriately, especially by being dominant/aggressive (nipping, etc.).
If you’ve ever had the opportunity to watch a group of dogs or a pack of wolves (video accepted!) interacting, you will see the dominant members use this corrective technique.
Grab the dog in a manner that will not cause injury to the dog or you (this depends on the size of the dog, it’s build, your abilities, etc.). Flip the dog over onto it’s back and place your hand on it’s throat (don’t choke it!). (Dominant dogs, of course, grab the throat with their teeth, but I don’t recommend that!) Stand over the dog in this manner and chew him/her out for 30 seconds or so. (This can simply be repeating ‘bad dog’ several times in a harsh, scolding tone.) If the dog tries to defy you, (struggling to get up, etc.) continue holding him on his back until he gives up (accepts your authority and position of dominance).
An accepting dog will pretty much go limp, drop his ears, tilt his head back to further expose his throat, maybe allow his legs to fall open to expose his vulnerable abdomen, and maybe wag his tail in a hesitant manner.
Most dogs will submit immediately, as this is an instinctive behavior common to, I believe, all canine species. Don’t immediately let go - hold him there a few seconds after he signals submission, then let him up. Immediately begin acting as though nothing ever happened - the dog will probably crawl to your feet and act extremely pitiful. I try not to make a BIG fuss over them afterward, just a pat or two on the head or a brief hug, along with an affection word or two. You don’t want to REWARD them for having been corrected, just reassure them that the matter is finished and forgotten and they are still accepted.
I wouldn’t try this with an adult dog that has a past history of aggressive behavior, especially a fairly large one, unless you’re very experienced and/or don’t mind the risk of being bitten before/until you can establish your dominance.
You’d be surprised at how will this works - you’re actually speaking ‘dog language’, which of course they understand much better than people language!