My mare is a small draft horse she eats free choice hay all day - I tie it up in a small hole hay net, like it seems you do too :)
She also gets Nutrena Grass Balancer - it's a pelleted formula that has all the vitamins and minerals that horses need, but no grain - my mare is fairly obese though xD
I find the best way to Up a horse's weight is the use of thoroughly soaked alfalfa/timothy hay cubes. It's about $15 for a bag of it, I give 2 quarts a feeding, 4 times a day on top of free choice hay and the regular grain feedings to our OTTB who's a very hard keeper. It just re inflates them, it also helps ensure they have enough water in the day.