I did email his owners back to see if he's still coming around there. I noticed when I walked by the other day that there was a carrier on the porch. Maybe it was for him.
If you see from the talk they had really abandoned him, there are two different solutions:
1. You try, together with them, to find him a new permanent home.
It is really their responsibility, and thus, best if they do it - but perhaps with encouragement and some help from you.
2. If that fails too, your reserve plane is OK. Ie having him as your outside cat. Quite many rescuers do have it so.
He is very much outdoors, probably not very cared for(??) by his owners (this you will know more exactly if and when you talk with them). And yet he has good flesh. A hypothesis is he does manage quite well.
And thus, he would do quite nicely as your outdoor cat.