I don't think he's being bad, he's being a puppy
Mine is 15 months and she loves to play and all of the time. What you think is aggressive is actually how GSD's play, they play hard. I don't have a yard so they play in the house, some people don't allow play inside, so the dogs go outside to play, but either way they are allowed to play. If you want to take off some of that energy, get involved in an activity...schutzhund, agility, nosework, etc. More walks, hikes, and maybe a flirt pole.