Remove the opportunities for your dog to nip and play rough by limiting the close up play. Play with your dog in such a way that nipping would be difficult, and keep yourself at a distance from your dog. So, no tug-of-war or wrestling or rough play. If you play rough, your dog will too. Instead, toss a toy or ball for your dog to retrieve, or do something so that he is not in such close proximity to you, so as to curtail the nipping. You can reward your dog with treats after play sessions when he responds the way you want him to. If puppy does nip, immediately stop the play session so he is not rewarded for this behavior. You can try playing with him again later.  Be consistent, and eventually puppy will realize that when he nips he no longer gets to play.

Dogs typically really want to please their people, but you have to teach them what pleases you by giving positive reinforcement when they behave the way you want them to.