Welcome! I am Dr. Altman, a licensed veterinarian and happy to answer your questions today.
Is he eating his own feces or another dogs?
This is called copraphagia, CLICK HERE for more information
In an older dog it can mean they are not metabolizing/ digesting their food well or it can mean they have issues with anemia or other organ function
If Dewey is eating his own feces you can put things like meat tenderizer in his food to deter the behavior
You can also purchase something to put in the food such as a product called deter
If he is eating other dog's feces this is much more difficult because you cannot treat their food!
In this case leashing on a walk temporarily to stop the behavior but if it continues I would definitely recommend having some bloodwork done and an examination to ensure there is nothing else going on that is causing this new behavior
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Please let me know if I can be of any further assistance or clarify anything today