How to train my pitbull?

Not sure where to post this. My pitbull is a sweet baby to family, but vicious to strangers when we're around. He was abused as a puppy. His previous owner tied him up to a tree and left him there for weeks with his food and water out of reach. We rescued him. He was also used in one dog fight. A family member got mad at us one day and brought his professional dog fighting pitbull over to attack us. That family member was kicked out of our lives. Our pitbull saved us that day and nearly killed the other pitbull for trying to attack us. We've tried rewarding good behavior and properly punishing bad behavior. We've tried to teach him how to socialize with other dogs and people, but it almost always ends in a fight because he goes crazy around strangers. We have played with him with his favorite stuffed toy. Whenever he starts to try and tear it up, we take it away and tell him no. Nothing seems to stick though. He also "escapes" occasionally. He has learned to jump and climb big fences. When kept inside, he usually doesn't get out until my brother isn't paying attention. The times we've had to run after him, he would come across people and dogs. He would be completely nice to them as long as we were at a distance, but he would try to attack others whenever we got close to him. I'm not sure what to do anymore.

Update: I don't have proof. Otherwise, I would report it. He's my step brother. Only met him a couple of times. He was trying to get back at our parents when this whole thing happened.