steal the dog and take it somewhere safe
That usually leads nowhere, as he probably would get another one within a couple of weeks time.
When I was still living in Australia, my neighbour and I were in a similar situation a couple of years ago, and we 'stole' a puppy, which was chained to a tree (the leash was attached to a higher branch of the tree, and the puppy wasn't even able to lie down or move away a bit to release itself!), without food and hardly any water 24/7.
At first we just went and fed and watered the dog twice a day as every single rib was showing on the poor thing. After a while we got fed up and took it for a daily walk too. One day my neighbour decided that this was pointless and just took the dog (which at that time was sick and pucking its guts out), to a vet and afterwards to her own home.
Less than two weeks later, this horrible person had another puppy chained to her tree. We realized that we couldn't go on with taking dogs home, or we would wind up with an animal shelter. So we informed the police, which just told us that chaining a dog 24/7 was not against the law in Australia. Then we called the RSPCA, but they told us that the only thing they can do was to go and talk to her, as they did not have any legal basis to stand on and besides, they had only one officer for the whole Northern Rivers area and we should try and solve the problem ourselves.
At least the local police-officer went and tried to talk to the person a couple of times, which usually resulted in the dog getting off the chain for a couple of hours, before it was back again. I don't know what happened to this poor puppy afterwards as I left Australia.
If I were you, I would first check whether chaining a dog 24/7 is against the law in the US (it is in Switzerland). If yes, it is certainly worth while to report to the police. If not, you might get yourself into a lot of trouble with our police-neighbour without being able to improve the situation for the dog.
Maybe you could call the local animal-shelter and ask them whether they have knowledge of similar situations, and what would be the best thing way to handle this.
I take it that the dog is chained in the backyard? Maybe someone from the shelter could 'per chance' visit you, see the dog, be outraged and turn in your police-guy in their own name?