My first GSD was stool eater and I've had other dog that did it too. I tried all the tricks in the book to make him stop - Forbid, meat tenderizer, and lots of other things suggested, but nothing worked. Switched foods, decreased the amount of meat, all sort of stuff. The best I could do was to follow him around with the scooper and get to the treat before he did.
My current GSD is Mister Clean, he won't even go near his stool, runs in the opposite direction after the deed is done. I think he would rather starve to death.
Some puppies outgrown it, hopefully yours will. If not, maybe the meat tenderizer or Forbid might work for him. You might also try dousing the stool with something really nasty like Bitter Apple or hot sauce and letting him have a taste of that.