Google "aggressive dog posture" images, look at the stiff raised tail, hackles raised etc.
I think you'd find your pup isn't as stiff as you think, and is just staring ready to spring into play action as soon as he finds someone who will play rough with him. He will not need much encouragement, maybe fast moving hands and feet, excited noises, laughing. They play really rough with their littermates. I had brother and sister last time, and they sounded as though they would kill each other! Most of the day would be spent play fighting.
I imagine actual aggression would be rare in puppies? But they are practising for being big dogs, and they learn through their play!
Wait for others with more experience than me though. I just hate to hear how worried you are.