I think my sicky chicky is on the mend. Brought him in the house just for a few minutes to syringe feed him some coconut oil/yogurt and hand feed him. His crop felt much less full tonight, but I have seen him eating all day so I expected it to be full. He pooped twice (why do chicken keepers celebrate poop SO MUCH?) and things seem to be moving through him at proper rate. I would say he's not back to full steam but 50% improved over yesterday.
So, who knows what it was. He is on the small side and so maybe his digestive tract is a bit sluggish. He really likes being in the house; we may have to get him a pardon from freezer camp in the fall.