I doubt that he was on his owner's stoop, because who would leave him for so long? And if they could leave him, then, end of discussion - HE's YOURS!! There is no way you could, in right conscience, return him to that situation. I once counseled my daughter to keep a kitten that she rescued from a front yard - the children had nearly drowned it in their wading pool; their mom yelled at them to take it out of the water, and they did, and it lay on the lawn half-dead; no care was given to the kitten whatsoever. My daughter called me at work, desperate, and I told her to rush the kitten to our vet & put it on my tab. Maybe it was governmentally illegal, but it would have been immorally illegal to NOT take him (and in that situation, I had to answer to a Higher Source). He recovered & was an awesome cat named Toby, who became my grandson's best friend for years. I didn't even check if the previous owners/torturers missed him - they didn't put up flyers, I know that. Had they tried, I would have notified the authorities; my vet was willing to testify to the near-dead condition of the cat, including signs of malnourishment.