it sounds like you have a nice big garden for him to run about in so its not too bad, at least you know your not the only one with a poop eating dog, which at least made me feel better at the time
, the only thing i can suggest is do what others have said and set up a few bait poops with tabasco on them, a few times of grabbing them might put him off, but their noses are so good i expect he will very quickly catch on,i think trainig collars are a waste of time IMHO . try not to get too worried over him doing it , its not nice, but weve all survived with our poop eating doggies