Your dog might be eating poop because of a rare illness called "coprophagia".
There are some medical conditions that may cause coprophagia, such as Exocrine pancreatic insufficiency, Pancreatitis, Intestinal infections, and more. Get your dog checked out, just to be sure there is not a health problem that causes this unpleasant habit.
Maybe your dog is bored and eating poop as a toy, another source of play. Especially dogs which have come from puppy mills or pet stores may see it as a toy because their former environment was so barren.
The emotional stress from a restricted or small area or being left alone for long periods of time without the companionship of the caregiver sometimes can result in the eating of his own feces.
Therefore, give your dog more daily exercise, interaction with his people and mental stimulation.
If you tried everything but nothing helps, educate yourself more about it, e.g. have a look here.
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