I would ix-nay the grain. It's harder for horses to gain weight and build muscle when being fed lots of starch and sugar.
I would instead go with lots and LOTS of grass hay or mixed grass hay. Weigh it and feed at least 2% of his body weight in hay a day, preferably more.
For his "grain" go with either a ration balancer (see Peggy's ration balancer thread) or a vitamin supplement designed for horses on no fortified grain. Uckele's
have been the best I have tried, in that order. My feed store only carries Purina's ration balancer, which I don't care to feed.
Add to either of the RB or the supplement 3-5 lbs of Alfalfa pellets or chopped pre-bloom alfalfa hay (the hay will mix better with the supplements, add some wated to the pellets if your horse sifts out the supplement). This will help build muscle and add calories for weight. Alfalfa is a forrage, and fed at the rate of 3-5 lbs a day, it will not mess up the balance of his feed.
Add 1-4 cups of whole flax or up to double the recommended amount of Nutra-Flax
. Ask you feed store about whole brown flax, usually sold in 50 lbs bags. Flax is high in fat for weight and high in Omega 3's for skin, coat, and hooves. It's a simple additive that won't mess up the balance of his feed. I have found flax to work better for skin, coat, and weight than corn or soybean oil.
This type of diet helped my hard keeper put on weight and keep it on, with a LOT less food than before. We no longer have to feed the flax at all, and the alfalfa has been reduced to just 1.5 lbs daily. He has a nice shine to his wither coat with the Uckele supplement. He is also calmer on this diet and has a better work ethic.