I found a lost dog who might have been abused, what do I do?

I found a little Boston Terrier wondering around my neighborhood about a week and a half ago. He was a real sweet dog and ran right up to me. I spent the rest of the night walking him around to see if I had any luck finding his owners, but never did. When I actually had the chance to inspect him, I realized he hadn't been eating and was all bones, his ears hadn't been cleaned in months upon months and he had an ear infection, he had some pus running out of his anus, and was covered in fleas. The next morning, I treated his ailments as best I could and started calling shelters, putting ads on craigslist, posting flyers and all that stuff, but haven't been able to find an owner. Tomorrow I'm taking the dog to the vet to see if he's chipped. Now here's my concern, his owners were very negligent and have obviously treated this dog terribly, if the vet identifies his owners, which haven't attempted to even look for him, do I have to return him? I'm afraid that the dog has to go back to a horrible owner and I want to fight for his safety.