It is better to feed hay by weight rather than by flake. 6 flakes of a lightweight, low calorie hay is not much compared to 6 flakes of a dense, heavy, hay. I would start by feeding him 20 lbs of hay per day. If he can't eat it all you can cut back. For a TB type 20 lbs of hay can be minimal.
You have to realize that a grazing horse may eat 1 and a half lbs of grass per hour. That may be 30 lbs or more daily. When you don't have grazing like that a horse needs the same amount of calories supplied.