Unstoppable biting

We brought home an 8 week old puppy two weeks ago.  He is a yellow lab/retriever mix.  The first few days he would try to chew on our arms or hands but we would give him a toy to chew on or we would yelp and stop playing.   In the past week though any time we try to pet him or hold him he instantly tries to start biting.  If we get up and stop play he comes after us and bites at our pants or legs.   He's biting so hard he's drawn blood on more than a few occasions. 
When he's chewing on our pants we've tried pulling him toward our legs to illicit the pull back response and that only makes him pull back only to lunge with more intensity the next time.  Or he'll go after our hands.   We try to stay calm and not move away from him so he thinks we're playing but he just keeps coming back at us.    We have him on a schedule where he goes into his crate for naps but I don't know if he's sleeping because anytime I check on him he's up an awake.    I have the My Smart Puppy book coming today so hopefully we'll get some insight.
I'm sure we've done something to confuse him about what he's supposed to do but it's gotten to the point where we almost can't have any interaction with him because it's too painful.  Any thoughts or ideas would be greatly appreciated.