Horses require about 2% of their body weight in feed daily to maintain their weight. So, if gaining weight is required they will need to eat more. My draft horses need to eat about 40 pounds each per day (they are all currently 1800-2200 pounds). Of course this is mostly hay, they each eat a bale of hay per day, but we do grain the ones that work more, or need it to maintain weight. In summer they are on fields, and we do not need to hay as much.
With drafts you need to be aware of Equine Polysaccharide Storage Myopathy (EPSM), and be cautious of feeding carbs. I avoid all sweet feed.
I do add minerals, flax seed (source of fat), salt, biotin (for hooves) and joint supplements to those that need it. If they don't need the salt that day, they don't eat it, it falls to the bottom of their bucket. We also always have a mineral salt block out in their field.
Each of our drafts eats a very different diet, according to their needs. We have some that don't get any grain at all. We have some that eat A LOT of grain to maintain their weight.
Just remember to use the percentage and make sure that they are eating at least 2% of their body weight to maintain and more to gain. It is important at first to actually weigh your hay and grain to assure that you are feeding enough.