I would take him to training at Petco. They will usually understand what works best for you and your dog. Some are treat motivated, some like toys, and some just want to be pet. As long as there is the keyword like 'yes'. I am very pro positive, and if you're getting a young dog any behavior issues you can easily correct with positive training before they get out of hand. It can be the same for older dogs or rescues, but if it's to the point where someone could get hurt, then you may need to try something different. Do your research, ask some trainers, see what works best for your dog. For example, our neighbors across the street have a very aggressive pit bull that would hurt my small dogs. In that case I would give her plenty of exercise so she wouldn't have any pent up energy that could be turned into aggression. I would stay out of areas with lots of people for safety reasons. I would be firm and say 'no' when I saw behavior I didn't like and re-direct her and show her what I do like. If she was still aggressive and could hurt someone or their dog, then, only after lots of training, would I maybe use a prong, but only for safety. I would still use positive training and correct bad behavior though, and find a trainer. Zak George has a lot of videos on aggression.