Thanks for all your replies.
Latest development: his head does not look any worse, but now (well, for the last hour) he sits alone and won't extend his neck out. He's all hunched up and when the RIRs were perched next to him to peck his head, I chased them away.
One of my little flock has been spitting up what looks like the Starter feed into the water dispenser, for weeks now. This afternoon, it looked like one not only spit up into the red part of the water dispenser, but vomited Starter feed all over the side of it, too!
IF it was the Polish rooster, maybe that's why he is not stretching out his neck and strutting around. He is just all bunched up. I wonder if he will even go into the coop at dark.
Based on your replies, I'm preparing a separate accommodation for him until I get this on track.
*sigh*
I hope this doesn't turn into a sad story...