Warm Bran Mash or Soaked Beet Pulp with applesauce mixed in? Or with crushed up peppermints?
A probiotic is also recommended. Prozyme is a good supplement that has digestive enzymes and probiotics in it to help restore the good bacteria in the gut that's been cleared out by all the rounds of antibiotics.
I don't know if yogurt is okay for horses? But that might be something that you could mix in with some food too.
"The EPM horse has special needs for nutrition. They should have very little alfalfa, and their diet should be rich in bermuda, orchard grass mix, and timothy hay. They also need help to boost their immune system, and we have recommended additional Colostrum or Beta Blend with Colostrum and Beta Glucan to help those horses, along with generous doses of EPM−Zyme which has anthelmetic herbs and herbs rich in naturally occurring iron. Our other clients have also treated their EPM horses by adding Vitamin E (from 1,000 to 8,000 IU per day) and vitamin C (1,000 − 5,000 mg per day). While others have added Folic Acid to their diets to help counteract the anemia caused by drugs." http://www.earthsongranch.com/catalog/epm.pdf
Also, what kind of grain are you feeding?