Firstly, whenever he bites too hard, make a loud yip like a puppy would make and end the play session off. You need to teach your puppy bite inibition. Try to take him to a socialising older puppy group or send him to a puppy daycare where he can learn to play nicely. Please give me a shout so that I can give you names of people in your area.
You can also try a spray bottle, and everytime he bites too hard you can spray him (with the sharp spray and not the mist) on his nose. Before you spray, say "ah ah" so that you can just use the words and he should stop the biting in a couple of weeks.
But remember to socialise, socialise and socialise
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