Sometimes when they get to that point there's not much you can do for them... it could be from the drastic temp changes but more than likely he has some other underlying health issue going on with him. Have you wormed your flock this season? He may have a parasite overload that has weakened his system, and the drastic temp changes were just too much for him... but it could be anyone's guess as to what could be wrong with him.
I'd put him in a crate, in the coop where the others can see him, with a heat lamp to keep him warm (but not too warm). I'd also add poultry vitamins, electrolytes and unpasteurized Apple Cider Vinegar (with Mother) to his water and offer him some scrambled eggs with his regular feed and see if he perks up, eats and drinks. If he seems to be improving with a little warmth and food then he might enjoy some meal worms for an extra protein boost too, if you can find some at a local pet or feed store.
If he is off his food/not interested in eating at all, then he is more than likely on his way out... you may be able to figure out why if you do a necropsy (or have one done) if he dies.
Best of luck with him, hope he pulls thru.
