My favorite feeds for putting on weight and maintaining a draft horse are Triple Crown Senior, alfalfa pellets or haystretcher, canola oil, Ration Plus prebiotic or a good probiotic such as Probios or Uckele’s AbsorbAll, and lots and lots of warm water, both in the food and in front of him. He also has 24 access to hay, water and a salt lick.
One dry quart of TCSr weighs 3/4 of a pound. You will have to weigh out alfalfa pellets if you decide to go that route in order to get the mix or your choice to 5 pounds per feeding. Someone here may be able to give you the weight of 1 quart of alfalfa pellets to make things easier for you. You can do several 5-pound feedings per day, which helps to boost the calories and weight. I would also consider that your project horse may need more hydration, which will also help to keep him warm, and look into some beet pulp sources. A fast, non-GMO source is Speedi Beet, which fully soaks in 10 minutes. Beet pulp is also counted on the fiber side of the diet.
I use canola oil in the winter with my horses. I consider my draft to be a big eater and a bit of a burner when it comes to food. He gets just over 1/2 cup per day. I used to use Cocosoya, but at $30 a gallon it was too expensive to keep up, so I go to the oil sales and get canola instead. He loves it.
My horses live in a place where it is cold and gets hammered with snow. To get weight on or keep it on a struggling horse, I take the daily total weight of feed and hay and divide it over three to four meals per day. By splitting it down that way you are able to keep the gut filled and them hydrated throughout the day and night, so they are continually processing food. I also add Ration Plus once a day. It works okay, but I will say that if ulcers are not an issue, at your point in the process I would consider using Uckele’s Absorb All or Probios instead, which will more powerfully help him break down his food in the gut, making it easier for him to digest as much of his nutrients as possible. Smart Gut may also be a way to go here. The more help you can give him here, the better off he will be.
Good luck with your project horse!