If he's biting too hard during play you should "YELP" loud in a high pitch, like another puppy or his mother would do if he bit them. I did that when my two were little and they would get me too hard. They would quickly stop and look at me...then lick my hand
as if saying sorry
They also were gentler the next time...now they don't even bite... I think they do with each other though because every so often I'll hear a yelp come from one of them.
You could try that....... It worked for me.