I'd be quite worried if any of mine went off food completely.

Have you tried adding sugarbeet? It may be worth using a probiotic or a feedbalancer such as Blue chip or Topspec so then you don't have a large amount of food for him to eat but you know he is getting it and then bulk the rest of his feed out with a chaff that he likes and both hay and haylage. You could also try Readigrass, I've yet to come across a horse that won't eat that!