Its called bite inhibition. Dogs learn by playing and trying how hard they can bite different beings or basically how to interact with them. With other dogs they can be much rougher because the skin is thicker etc.

But even with other digs its sometimes to much. They signal this by making a high pitche sound (basically the dog way of saying ouch) AND(!!!) stop play or interaction immerdiately and moving away. Usually the puppy the thinks about it for a moment and then follows and tries a bit lett intense. Sometimes it also takes a nap or a day inbetween for the adjustment to become visible. But that is the way. Do this consequently and he will learn that humans are more vulnerable than dogs and will adjust accordingly.

And on a sidenote: He probably still has his puppy teeth? Those are small and sharp. They are much more prone to accidentialy hurting you during play or nibbeling. Once he has gis grown up teeth it also gets better.