I had a OTTB gelding I got when he was over 300# underweight. I started him on Farnam Weight Booster and a mixture of Purina Senior and Purina Safechoice, along with free choice grain. He did wonderfully and put on weight like crazy. You may also want to try giving him his grain in a mash rather than hard, it will be easier for him to chew and digest that way. OTTB’s eat a lot more than your typical horse due to accelerated metabolisms. Make sure you’re giving him a fair amount of grain in proportion with his body weight and his height. Best of luck!