I have also heard pineapple. Some dogs or maybe most, have the ability to smell out undigested protein in feces, both their own or other dogs. They have the urge to eat it because they themselves may be protein or B1 deficient.
The pineapple works for them as it does for us, it allows the body to absorb more protein, hence leaving less in his feces, he can't smell it, doesn't need it, and hopefully won't eat it.
Other than that I believe there is a product you can add to their food to make their waste unappealing.
Solid Gold S.E.P. (Stop Eating Poop) Dog Supplement I also had a friend use meat tenderizer, the dog stopped eating his own, but still ate other dogs' waste they encountered on walks.
Good luck, I can imagine how frustrating it may be.