I've been feeding Cur-Ost for a few months and have been quite happy with it. My horse was eating it well mixed in with his feed. HOWEVER a couple weeks ago I moved him from a dry lot to a 20+ acre pasture. He is thriving and stuffs himself with grass all day and wants absolutely nothing to do with any grain or feed. He will take one bite then just is not interested. So that means he's not getting his cur-ost in him. Is it possible to mix it with something and just syringe it into their mouths? Applesauce or some sort of oil or something? IDK. Or a way to turn it into a "treat" that I can hand feed him (cause he still loves treats!)
My mare does that when she has an ulcer flare up. And green grass will do that to her in a few days due to the richness. Not sure if your horse is an ulcer prone horse but in my experience when they turn away ration (especially if they've previously eaten the same thing no problem) they have a sore gut.
Sorry -- not much help in ways to get it into him without grain :(
My horses LOVE their Curost products, but they aren't wild about eating it dry. (Kind of like you eating a bunch of powdered milk or equivalent.) I have to add a little oil or water and then they lick the buckets. You might give that a try.