Not dominance or agression -- your pup's trying to communicate with you and barking's the only way he knows how to at this stage. Think of it like: "But mom, I WANT to chew up the couch, chase the cat, sleep on your bed." If you're consistent with the "No," he'll eventually become resigned to the fact that certain behaviors are not going to be tolerated and will stop "sassing."
Della - GSD- DOB 10/23/13
Dina - GSD - RIP 09/01/98