I might be wrong on this but what I have learned is 6 1/2 wks is still pretty young. They learn from mom and littermates about play biting. When mine used to do this alot I would yelp, normally that didn't work so I get up and walk away for a bit. He usually gets the hint that mom doesn't like it. He has been doing alot better and it only takes a NO BITE now. Good luck
Dani