My wife and I just rescued a 3 month old stray border collie. He was fine for the first few days, but now he has taken to extreme biting. Any time we take him for a walk or want to play with him he starts to jump at us and bite at our pants, socks, shoes, hands or arms. Some times he wags his tail and some times he doesn't. He is biting us really hard and we've followed all the advice that we've seen including yelling "ouch" and "aaah", but he doesn't even notice. We definitely can't ignore him because he doesn't stop biting if we turn around and try to ignore him. Also, we can't walk away because that further fuels the fire. It's so bad that if we have him on leash and he starts doing it we have to shorten the leash and hold it away from us to avoid being hurt. He's so aggressive that he doesn't even care that the leash is tight on his neck. We've enrolled him in puppy classes starting on Monday, but I'm afraid he'll hurt the trainer or another dog.
Please help us. We don't know what to do.