Found this online. Perhaps this will help?
I myself am curious to why your boy is doing this.
Myths and Wives' Tales of Feeding Horses
"some truth some fiction"
Stephen G. Jackson, Ph.D.
Kentucky Equine Research, Inc.
Versailles, Kentucky
Horses that Practice Coprophagy are
"missing something"
I do not think that anyone knows why some horses eat their feces. There are probably several viable explanations. One could really assume that from an evolutionary standpoint this is normal rather than abnormal behavior. It kind of makes sense that the horse would view feces as another way of obtaining nutrients (ie extraction of nutrients from the feces or recycling nutrients such as microbial protein).
There are several instances when horses are commonly observed to practice coprophagy. The very young foal may be seen eating the feces of its dam, probably a method of inoculating the gut with bacteria. The horse on limited fiber or caloric intake, may eat feces to fulfill a need for fiber and/or calories. In either instance, supplying adequate feed and or fiber may or may not cause a cessation of the behavior. Most frequently adding fiber to the diet yields the best results as far as eliminating the habit of eating feces. Many times we find the show horse, especially the easy keeper on limited feed intake and on a sporadic and inadequate exercise program, to be more prone to coprophagy. One must think that this habit may be behavioral, and primarily a result of boredom and in some instances have may absolutely nothing to do with nutrition.
