Whenever you get a starving horse, the first step should be a complete physical from a vet that includes pulling blood and checking for compromised organs----if the body is starving, it will pull stored energy from anyplace, including vital organs. UC-Davis, considered one of the best on feeding starving horses, recommends alfalfa since it's nutrient dense and the high calcium levels act like Tums for humans if the horse has ulcers. Move the horse up from many small meals per day to free choice hay. KISS works for feeding horses----rather than feed all those extras, buy a good quality Senior Feed and slowly, over 2 to 3 weeks, move the horse up to the amount per the feed tag for his ideal weight, split into at least 3 feedings.
When a starving horse is recovering, the body first builds fat around the vital organs so it's very hard to judge if he is gaining without using scales (My hubby hollered at me to get off the scales when we weighed our starving rescue 1 month post rescue---we couldn't see the weight gain, but he sure had gained almost what I weigh!) Once the horse is the proper weight, it can take another year of riding and working the horse correctly to build condition, top line, and muscling.
Enjoy the adventure of giving a horse a better future!