My puppy has been biting his leash from 8 weeks. This can be embarrasing in publice places. He bites and growls. He's very submissive though. When he sees us or other people, he's ears go back (its not the fear kind). I have started obedience class but this is giving me a headache. Obedience helped with other things but not this. When he bites it I tell him sit, but he won't listen. I think he's trying to push me. What could I do?