I know foals will eat manure to help build up the microflora (probiotics) in their hindgut. You may look into using a good probiotic for a while and see if he quits.
I have some good products and articles on Omega Horse Solutions
you may look at to get an idea of what to look for. My two colts are on the dac 911 - the new one is on it because I am changing things around on him and I know it is causing some stress and my 2 yr old is on it due to the recent move from SC to Fl. I will leave them on it for 1 month and then switch to dac Digestive Aid (my 3 yr old filly is already on it.)
There is at least one digestive article in the articles link on the site.