My 2 year old pure black lab, otherwise adorable, is afraid of strangers and has a fear biting problem. He nips at the ups man, the repair man, etc.,,,,,,I don't know what to do. I just put out a bunch of dog bones by the gate so that when someone comes, he can throw a bone to Pepper. When they approach him he gets scared , stiffins up, barks again, getting all worked up, and nips then runs away. He has never made physical contact, just the motion with his teeth bared. What should I do? He is a sweetie once they get to know him but who has time for that? HELP