I have an American Bulldog/Pit mix taken from her mom too soon as well (got her at 4 wks old, now 10 wks old). it's... a struggle to say the least... to stop her from play biting. "The Bite Stops Here" didn't work for me at all. Saying ouch would only make her more excited. Leaving the room wouldn't work because she would follow me. Pushing her away made her more excited. Ignoring her made her more excited. So I had to come up with my own style. She still play bites but not as hard.
I redirect her mouth at ALL times from my skin to a toy even when it's inconvenient for me which means I now carry a toy with me around the house in case the puppy dive bombs my ankles. I redirect her mouth to the toy during play, too. Eventually she gets bored with me putting something in her mouth that she doesn't want, and then she wants to play a new game. Keep your puppy occupied at all times with lots of games so he's not always frustrated with his teeth. Tug of war (this is a good time to teach "out" or "drop it"), chasing each other around the yard, teaching new tricks, short-distance fetch. Keeping him occupied will also prevent him from chewing on your furniture and keeping him out of places he's not supposed to be.
Also, give him something CRUNCHY to chew on! Large carrots are cheap from Wal-Mart and really effective. Apple slices/core with seeds removed. A crumpled plastic bottle covered in a towel works. A rope toy. He is teething and his teeth hurt, which means he's going to bite you or anything as hard as he can to relieve the pain.
Good luck!
