I was at a dog nutrition seminar recently and the speaker had some ideas. She said to add acidophilus to the diet for one week, then up protein a bit, then try adding some vegetable matter. Now that I read my notes I see that I did not write down if these are exclusive of each other but I think you add one on to the next if the first one does not have the desired effect. She said no guarantees but it has worked some times. In the meantime you must be strict with freedom and use the leash.
Are you teaching LEAVE IT? It worked for Oban but he was not a hard core poop eater. Just the occasional horse poop or dead fish. While he was learning what things Leave It applied to I carried very high quality treats with me, real roast beef. If he left the horse poop then he got a jack pot of roast beef. I confess/admit the little devil would go right back to the horse poop unless I continued dispensing the good stuff down the trail.
ETA: Oban was older. At that age we had feet upon feet of snow and horses were not on the trail as it was used by snowmobiles. I think the fact he didn't even see horse poop till he was about 6 months helped. He was a bit past that baby put everything in the mouth stage.