I think it is great that you are trying everything you can to help this adorable tabby you named D.O. Bless you.
I agree with the others, if it isn't a problem for you, to keep him in the garage. At least he has some shelter there from the cold. I would like to make a suggestion if you could, to put some straw in the box for him as this will provide more warmth for him. Also outside cats will beef up when the cold weather hits whether they eat well or not. It's amazing, as we have a few strays around our area also and in the summer they are so thin, but come the cold weather, they almost double in size.
Good luck in trying to find a foster home or no-kill shelter for him as he is a keeper.