He will most definitely need grain if all he has is hay and no pasture. There is no way a 4 yr old, 16.1 hand appendix in work can get all the calories he needs from just hay. And more than likely he hasn’t been getting the proper concentrations of vitamins/minerals that he requires either from just hay. I would imagine he would need at least 3 lbs of grain per day along with hay. Most commercial feeds have added vitamins/minerals. Buckeye Nutrition, Triple Crown, Nutrena, or Purina all make good feeds. Buckeye also makes a fabulous fat supplement called Ultimate Finish. You will see a lot better results with that than you would with Cool Calories. But again, getting him on a grain is going to be the best way to get weight on him and to insure he is getting the proper nutrient concentrations.