It's possible that he/she is starting to teeth, and it brings them relief to bite down on things. People will suggest a lot of different solutions to the biting problem, but for me I just yelped (couldn't really help it!) and pulled her off and then would start again. She learned to associate biting with not being able to eat, so she stopped.
It could be a few reasons.... your let down might be forceful and he trying to stop the flow. Possibly starting to teethe. Or maybe he's done and doesn't know how to unlatch.
Either way... when he does it... tell him "NO" and take him off for a minute and then let him relatch.
When my son had no teeth I would just keep telling him "No" but after get got teeth... if he bites me more than 2 times he's done and we try again later.
Probably teething. That's what mine did. Have fun we he gets teeth. Mine has teeth and she only bites to get my attention. Plus, before she bites she gets the most evil smile on her face so I know when its coming if I'm paying attention.