Sure, lineage/breeding, I get. But I have never heard of, or read a case of, a horse that young having symptoms that bad from PSSM. That’s my source of confusion.
Se-deficiency would be much more likely BUT, if it was to the point he was that uncoordinated, I would expect to see mane/tail falling out, and feet to be having major cracking issues.
Not all EPM horses fail typical neurological tests, even as they walk “drunk”
The “fly twitch” test really only tells you something if he doesn’t react. If he does react, it doesn’t mean there isn’t a problem.