Gigi did that too. Well, she didn't do it much any more. I believe it's one of her many "stages"/"phases". And a "no" sometimes means a "blah" to her, in my opinion. She doesn't care. Maybe you can do a time out. Whenever he bites (painful or not) yelp for pain (exgerrate!!!) and storm out of the room angrily and the return after like 20 seconds. May be he will get it that biting mom will makes mom leave the room.

Good luck.