Turn him in, that's all you can do. People like that shouldn't be allowed having any contact with animals.

My dad once had the police involved when 2 horses were without water for days when it was very hot in the summer. My dad refilled their tanks after seeing their situation and called the police. The horses were taken away the same hour.

I'm sure that if you mention your fears to the authorities, help and advice will be provided. You won't be the first. Even if other people don't see the dog, they usually know that there is one.

I wished, like as when you adopt a child, someone would come over every once in a while (unexpected) and check the animal and its surroundings. In Germany we're all registrated as we have to pay dog-tax. It's easy to check on animals that way, and I wouldn't mind anyone coming in so that cases like you describe never happen.