1. Rice bran will provide him with cool energy and not amp him up. Tractor Supply carries Manna Pro's Maxi-Glo.
Some stores carry the meal form, others the pellets; I prefer the pellets and the horses loves the taste.
Rice bran is high in fat (22%) but it's healthy fat; my senior EMS horse has to have it to keep weight on him.
2. A high quality ration balancer to provide vitamins/minerals. Unless you can get EquiPride, a soy-free vitamin/mineral supplement that is fed at only 10 ounces/daily.
3. Brewers Yeast to help support the digestive tract at 2 Tablespoons twice daily<---that from my vet. The product I buy for three of my four is Diamond V-YC; it's in a 50 lb bag and only around $19. I have also heard brewer's yeast is great for hooves.
3.1 Or, if you can afford it, either get EquiOtic from your vet, as it's prescription only, or buy Succeed. Both are microbial products that are fantastic to support the entire digestive tract, including the hind-gut.
Both are fed daily.
Succeed is in paste form and the horse will take the tube from you, it tastes that good.
EquiOtic is in paste or powder. I asked my vet which was more efficient as the ingredients aren't quite the same between the two. He replied the powder is more efficient so that's what I feed.
4. High quality grass hay. Some alfalfa mixed in may or may not be ok; it just depends on the horse and how old he is.
My two mid-20's Fellas that used to not be able to sniff alfalfa without getting bug-eyed, now get two pounds daily of well-soaked timothy/alfalfa cubes.
One of them has Equine Metabolic Syndrome and the alfalfa has no effect on his insulin/glucose levels.
This is about as bland of a high energy-yet-cool-diet I can think of. Maybe others have some suggestions:)
Hope this helps