Generally horses that won't put on weight with feeds or supplements have one of four problems, or a combination.
- Illness or Disease
- Parasite Infestation
- Bad teeth
- Cannot properly process a diet high in starch/sugars
First I would have the vet out to give him a good once over. Have him check his teeth, fecal count, and maybe run some blood work. Make sure the horse is truly healthy.
If all that checks out, then you need to alter your thinking on horse nutrition.
My husband's Anglo Arabian was the same way. We fed him all kinds of things and nothing really helped. We did finally stumble on a solution: STOP! Go back to "simple" in his diet. See, with these high energy horses or "hard keepers" in general, the more starch/sugar you give them, the more it revs their metabolism, which in turn makes them burn more energy, which makes them require more food!
So, STOP! Go back to just hay, lots of hay, more hay than he can eat. Just plain grass type hay. Add to that lurcene chaff (chopped alfalfa) or lucerne pellets (alfalfa pellets), a good vitamin supplement, a digestive aid (probiotics), and a fat source (I prefer flax or rice bran, but oil will work if you're on a budget).
Start with 2 lbs (about 1 KG) of lucerne (alfalfa) a day with 1/2 cup of fat. Keep him on that for 4-5 days, then increase to 3 lbs (1.3 KG) of lucerne/alfalfa a day plus 1 cup of fat. Keep him on that for 2 weeks and see how he does. If there is no weight gain, then increase to 5 lbs (2.25 kg) of lucerne/alfalfa plus 1.5 cups of fat. Keep him on that for 30 days. You should see good improvement.
Once he's up to a good weight, pull back to the starting amount of 2 lbs (1 KG) and 1/2 cup of fat. Remove the digestive aid (probiotic), but continue the vitamins. Adjust the amounts as needed.
TRUST ME on this one. The simple diet WORKS for hard keepers or high energy horses. Your horse will gain and maintain weight on a LOT less food than you thought possible. The key is to start with QUALITY hay, and lots of it. The lucerne/alfalfa adds protein, quality amino acids, and concentrated calories that are a "slow burn" and won't hype up the horse.