Thanks so much for the additional info.
Update ... We never found anyone to do a float for us and couldn't identify what we were seeing in the home fecal float. But, our chick is about 80% improved today!
His energy levels were better by yesterday afternoon and by this morning he was seeming almost normal. Good enough that we put him out in the dog crate in our garden next to his buddies for the afternoon. We brought him into the brooder again for the night - and I've just seen that his poops look almost normal!
That supplement we got at our local farmer's supply says it can be used to treat necrotic enteritis and has antiseptic, germicidal and astringent properties. I don't understand what active ingredient is working and what it might indicate we're dealing with. The store said they suggest it for coccidiosis but local hatcheries I've talked to have said that cocci would be unusual in our setting. Very small flock, large run, wet climate but generally not that warm relative to the States. But maybe we had a perfect storm. It was awfully wet here a short while ago.
I will update if there are any drastic changes but I'm hoping to get Topaz back with his little flock of cockerel buddies in 2 days - and plan to treat him and that whole little flock for 4-5 days to decrease the odds of a recurrence and address some watery poops I'm finding in the gang outside (though their energy levels seem fine - they're fighting like crazy).
Now if only this supplement/medication could transform even one chicken out of what's looking to be a 100% cockerel hatch into a pullet!!!