I agree, Marek's is a good possibility.Yes, it works in a pinch.. maybe offer some straight and still plain water on the side.
Did you get a chance to check his crop condition at the end of the evening and in the morning before he eats and drinks again?
Yes, I was also thinking this is a possibility as well due to being a new property.
Still having an apatite is what leans me towards... possibly marek's. Just recently switching form starter... *shouldn't* equal vitamin deficiency now...
Since I have suggested supplementing... please do NOT supplement vitamins WHILE treating with Corid... if you try that route as they negate each other.
Since this is bird is a several months old, he may still have a bit of appetite with an overload (unlike a few weeks old chick). I would think he likely has some resistance to at least one strain of Coccidia, so resistance may be helping him out if there's another strain at work here - just one of my crazy thoughts/speculations (LOL)
I would give the vitamins/electrolytes until Corid is started.
No, just a few days of switching feed wouldn't have caused deficiency, but birds are very good at hiding symptoms - so it could have been going on for a while unless it is sudden onset Marek's.