Sometimes eating poop is instinct, sometimes it's because they're not getting enough of a certain nutrient, sometimes it's because they're not digesting things properly, but sometimes it's just because they like eating poop.
Get the dog to the vet to see if it's a nutrition or digestion issue, and if it's not, then you can try to get him to stop eating it with one of these types of things http://www.petsmart.com/product/index.js...Id=4299172
My dog slowed down on eating his poop once he was given new foods that he could digest easier and a multivitamin each day, but still when he's bored or particularly hungry, he'll go back to his old habit.
