Is this a good diet plan for a miniature horse?

As the year goes on and there is less nutrition in the gr I would introduce some hay to him gradually. You'll know if he's hungry if the hay you give him disappears as soon as you do!! It's the same with feeding a hard feed or mix, do it slowly so that his body has time to adjust. You will find that once the spring gr appears he will leave his hay and you can leave out his hard feed too