Believe it or not, they eat poop because they think it tastes good. There is a lot of flavor and the smell of the food (especially if it is high quality) comes out in the poop.
You will need to retrain him to eliminate the behavior. You should start by picking up the waste WHEN HE LEAVES IT. Keep a pack of doggie waste bags with you and dispose of them appropriately. This will take away the temptation.
There is also a product that you can sprinkle over his food called FORBID or DETER that will make the poop taste terrible. You can get it at your vet. Adolf's meat tenderizer that you can get at your grocery store will do the same thing.
This is a nasty little habit and it will take some work to get rid of it.
Let me know if you have more questions.
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