It's puppy play nipping and biting. I think he wasn't with his litter long enough to learn real bite inhibition.
careful. He may nip at a dog that isn't real tolerant (you never know at a park) and then your pup would become dog agressive cuz of the bad experience..especially if it is a prone breed. My pit, for instance, I gaurded extrememly carefully as to the other dogs he was exposed to, as they are prone to dog agression. One bite from another dog and he would have been ruined.
I did better finding friends/family with veryyyy tolerant dogs for him to meet. Dogs that would let him know nipping isn't ok, but who wouldn't harm him. Also, when he was a pup I never let him bite flesh..not playing or anything. he bit any flesh I would tell him NO and redirect him to a toy or something to chew on. Worked great..he is a total handful and a PITA now..BUT he doesn't bite or even mouth people/flesh.