I think what you may be experiencing is a little puppy testing, and I have seen similar situations turn into aggression, as well as severe dominance. My own Boxer, Jakob, was VERY nippy as a pup, and two things worked well.
The first thing we tried was suggested by our vet, and that was to hold the muzzle (gently, but firm) with a good, solid NO. It suprised him and gave us the opportuinty to give him a toy if he wanted to get rough with something.
The second thing was the "long down". Since almost EVERYTHING else we tried made him fight harder, (BOXERS- go figure!
) was to simply require he lay in a down position for 5 minutes, like a "time-out". Even if I has to stand on that leash to keep him in a down, it was much more final, and didn't leave us open to manipulation on his part.
This behavior kept up for a few months, so be patient. Also, you could research Patricia McConnell and what she calls a "body block". That works MIRACLES with dogs that jump or get pushy.
-Renee & Jakob