Morgans are the poster child for IR (insulin resistance), he is likely IR and that is probably why he responded to well to the low s/s hay. Like so many things, they don't all follow the rules, and while his symptoms might not be typical, that doesn't mean he isn't IR. I'd go for the blood tests for IR, so I believe that's glucose, insulin and leptin. I'm sure someone with more experience than me will let us know if I've steered you wrong.
