Hema is right, he is teething. You must teach him that when his teeth touch skin, play stops.
From now on, as soon as his teeth touch your skin, give a loud yelp like a puppy. Make it high pitched and quick. He will stop immediatley. If he bites again, do the same, this time do not engage him in play once again. You get up and you leave.
He will learn that when he bites, play will stop. If he wants to keep playing, he will stop biting =D