You know it's possible the old dog wandered off - my friend has an old cat that has feline-Alzheimers, she's always wandering off. Maybe this ol'guys mom & dad will be looking for him and he'll be reunited....prayers coming his way and all paws crossed. As far as him seeming mistreated, it could be that he has sight and hearing problems, too. My Sherman always ducks and gets skittery when people reach for him, and it's because he has cataracts and doesn't see very well.
You did the right thing. <<<hugs>>>