I'd be a tad bit concerned but I wouldn't let it get in the way of adopting him. One of mine used to nip as a pup. He never nipped me but constantly nipped my sister and brother. It's definately a habbit you can break. It sounds like he's wary of strangers and doing a bit of fearbiting. Tell any strangers that stop to pet him to let the dog aproach the stranger. The dog will decide when it's ready to be patted on the head.