FYI-- the rooster walked into my open coop a couple days ago. I have a low fence (with lots of ways to climb out, my girls with clipped wings still occasionally hop the fence if they see something more interesting on the other side) and he has chosen to stay. He integrated into my flock like he belonged there, so I have a feeling he'd been hanging out outside the fence talking to my girls for a lot longer than I knew!
I really don't think I stole him, in fact at this point I think I saved him by letting him in. It's below freezing outside with over a foot of snow this week :/
Perhaps you have helped him by giving him a temporary home during the hard weather. A neighbor (perhaps one of the two houses you mentioned) may be looking for him.
You haven't proven that he was abused, simply misplaced. That's awfully like stealing in my book. Take care of him, for now, but as soon as the weather improves, try to find his rightful owner. For all you know, they will say, "Keep him if you want."
I'm not saying the rooster doesn't belong there with you. It sounds like he likes it. I agree we should always do what is best for the animal, but to blatantly ignore the human element (his owner) isn't right. You haven't followed through with the courtesy of at least giving any attempt at finding his original owner. That is disrespectful, IMO.
You might not agree with me, but you did start this thread asking for us to give you feedback on if we thought you should be able to keep him. I'm not saying I don't think you can't keep him, but I don't agree with the way you're going about it.