Adding one thing the thread has danced around: respiratory infections are more common in cold, damp, badly ventilated housing, and they present exactly like this at the start. Off their food, quiet, sitting hunched in a corner.
So while you are checking the hay and the water, look for sneezing, crusty eyes and any noise when he breathes. A shed in a cold snap ticks more than one box here, and the vet will want to know which of those you saw.