Have your vet test him for it, that's really the only way you know for sure.
Have you tried putting him on a probiotic? That helps them absorb all the good stuff from their food instead of pass it on out with their manure.
Also do you free choice good quality hay and salt?
I don't know/have not heard of any of those feeds before. But my horse bloomed on Triple Crown Senior. It really helped him and I recommend you look into it. It's a high fat feed but it has all sorts of vitamins and minerals in it.
Have you had any tests done on this horse to see if he is insulin resistant or deficient in anything? Any kind of deficiency can make it very hard to gain weight.
Hope that helped.
"Strength is the ability to use a muscle without tension"
Last edited by Skyseternalangel; 06-19-2012 at 08:05 AM.