If you are looking for a grain supplement, try Max-E-Glow, by manna pro. It is a rice bran, high fat supplement that does NOT make the horse hot. I used it on my arab and he gained weight quite quickly (granted he was only 4). This is what my barn owner recomends and it works wonders.
I will check with her tonight to see what she feeds her 27 yr QH who has heaves and is hard to keep weight on. But he looks quite well for all his medical problems. I do know though, that she feeds him equine seinor, and because he hardly has any teeth, she also feeds him alfalfa pellets.