Could this be a stray cat? What should I do?

I had a rough looking tom cat come into the garden and he seemed ravenous, i made a paper collar and sellotaped it together after writing if you own this cat please phone (number) if no one phones or the collar is still there in a day or so, then take it to your local vets to be scanned for a chip, it doesn't cost anything. The tom cat still comes to my house but he lives a few streets away and i know his owner, he is well cared for but prefers to be outdoors, hence the rough coat, he had also just had an op for a collapsed lung so you can't always tell by appearances.