My pup has strange behavior when working with the tug. He is also very nippy and grabs at my pant legs. What should I be doing?

Hello, I recently purchased some of your DVDs; Building Drive and Focus, Bite Training for Puppies, and The First Steps of Bite Training. I plan to follow up with the first steps of defense and training personal protection dogs. I find them very informative and very well done. I have a couple of questions that I would like your insight on. The first is I have a GSD that is 20 weeks old. He is very smart, obeys commands sit, stay, retrieve, leads well sits in the kennel while I go out first and sits until released sits and waits to be released to eat his food seems to recognize me as pack leader and has a lot of prey drive. But he is very mouthy. He takes every opportunity to bite me on the hands & arms and as I walk he bites my legs and tugs at my pants. When I grab him by the scruff of the cheeks stare into his eyes warn him and even growl at him sometimes he will start to circle me with his ears pinned back barking and snapping at me. Also a couple of times when I try to get him to release the bite tug he has growled at me. Then I put him down, place my fingers into his neck and growl at him. When I let him up he is ok. The other thing is when he counters on the bite tug and I release it he walks off and stands or lays down and starts hunching as if he is breeding a female. My next DVD will be dealing with dominant & aggressive dogs. E would appreciate any advice you can give me.

Thanks,
Greg