Having rescued more dogs than I can count, I understand and share your anger. My first step would be to spend a couple days getting photos as proof of the dog's abuse. This makes it harder for people to lie to the authorities. During those couple days, you could try to gain the dogs confidence. Treats first thrown over then handed through the fence can help. Avoid eye contact when possible so as not to seem threatening, and keep as low to the ground as possible. Use a quiet and soothing voice to talk to him. Visit as many times a day as possible. If there is any response from the dog over this period, then it probably isn't too late for rehab. A "hopeless" dog probably won't accept food, so even if he barks, growls, etc. but still takes the treat, it's a good sign. When I've had to bring authorities in, I always call the police as well as the appropriate animal authorities. Once the dog has been taken to the shelter, go visit right away. Keep in mind that the dog feels that his life has just made a turn for the worse, and seeing a person that was kind can help lift his spirits. Take the same treats that you were offering over the fence. Use the same kind voice. Aggression in dogs is almost always rooted in their fear and not in animosity toward us. If you can break through that barrier, then the dog has much better chances of finding a good, caring home.
I know that I'm really simplifying here, and people have told me that I tend to sugar-coat the rehab process, but I have gotten many dogs out of similar situations using these basic guidelines. I've run across very few that can't be helped. Unfortunately, pups that have been raised in this situation are almost always the "hopeless" ones. But even if there's a glimmer way back in the dogs mind of people at one time being nice, there's often a way to find it. The downside is that it can take years to undo the damage.
One thing I've found never to do is confront the owner. Often anger meant toward you will end up directed toward the dog.
I won't go on and on here, but if I can help, feel free to post again or PM me.