He's not a puppy anymore, but he isn't an adult yet either. He still has some growing up to do and he needs your help to develop into a well-adjusted adult dog. It helps to be familiar with the aggressive behavior that may be displayed during this period so that if your adolescent puppy shows this behavior you will recognize it, and know how to best deal with it.
Signs of aggression
Watch for the signs of aggression. Some signs may be subtle but if you aren't paying attention, and miss the initial signs, your puppy's aggressive behavior will get worse. For example: you have a guest over and your puppy jumps up on the guest in greeting. You command your puppy to sit and he instead leaves the room. You may rationalize that although your puppy ignored your command his disobedience wasn't really important because the same end was achieved – he stopped jumping up on your guest. But in your puppy's mind he has just won a battle and he may push the boundaries a bit further the next time. Never give your puppy a command unless you are prepared to enforce it.
If your puppy becomes possessive with his toys and growls if anyone goes near them he is showing aggressive behavior. Or maybe your puppy has a spot on the couch that he's claimed as his own and he growls if anyone else sits there. That is another example of aggressive behavior. Testing the boundaries he has been given as a puppy is normal in adolescence but it's important that your puppy's behavior is corrected in a consistent manner.
Don't make excuses for your puppy's behavior. Excusing his aggressive behavior because "he never did that before" or "he's always so good with me" will only make the situation worse. Watch your puppy for signs of aggression and be consistent in your treatment of it and you will stop your puppy's aggressive tendencies early. If your puppy's aggressive behavior is not caught early it may be necessary to enroll your puppy in obedience school.