Rob, alot of horses that have never been given grain, especially young horses, don't know what grain is. They don't understand the smell or texture of it. You can buy the Probios paste for cattle and it is the exact same stuff as the equine. Only it is blue in color, but if you compare the label it is the same ingredients. I would just keep putting it in front of him in small quantities and he will eventually start eating it, just takes some time. Keep it in front of him all day, change it if it gets stale, but he will start eating it. I have babies come in all the time that don't know what it is, just because mom never got it and they never learned to eat it. Alot of Sundance farm are like that. It takes them time to learn to eat it. Careful on the fast track, as it can cause diarrhea.
Good luck.