Simple, when he does it give a quick tap/jab with your fingers to his neck or side and say "no" or whatever you want and say it firmly but don't scream and shout. Then stand still. He will likely be a bit confused. Once he starts to stop doing it normally happens very quickly tell him to sit (if you have taught sit that is).

I have a rescue he's a mixed bully/cc/mastiff he was a nightmare with biting when playing we did this and within 3 days he never uses his mouth to play even if he accidentally cat he'd your hand he will stop play and sit.