I feed an oatless grain to my horse, As he's a thoroughbred and he can lose weight pretty fast.
I find that extruded barley works wonders too!
Maybe try adding some epson salts to his feed, to help with magnesium levels, that'll hopefully help him become less anxious!
I have a very hot horse, who loses a lot of weight through winter, so it was difficult finding the right feed! But I'm really happy with my barley, and chaff.
Some possible things to add to his feed which I find helps with condition are,
Sunflower seeds, really good for fattie oils, and Oil! (I feed canola oil :) )
Hope it all works out for you :)