I still wouldn't bring him in. By then he'll have adapted to the cold and bringing him in will just make him uncomfortable. Think about this - do you have wild songbirds? They're a lot smaller than your chicken. They survive just fine, even without a coop to live in. Let him hang out with the other chickens and they will be fine. If you want, maybe put a thick layer of straw on the floor of your coop for some insulation and for them to snuggle down into for warmth.
