Why does my kitten bite my hand when i'm just petting him?

my kitten seems to think that when im petting him its playtime and he starts biting my hand. The bites used to be simple gnawing but as each day passes his biting is becoming harder. I used to just play with him when the biting was not too hard but now it feels like he might break the skin. How do i make him stop this so i can just pet him? Or is this a phase or something? Does he do this because he has no other kittens to wrestle with? What can i do?