As can be expected, this symptom is particularly disturbing for doggy parents. The ingestion of fecal matter, coprophagia, is a behaviour that is yet to be truly understood.
Some vets believe that dogs' highly sensitive sense of smell lures them to some of the most unusual and often disgusting odours. Others believe that dogs will consume their own feces because they may have a digestive disorder or some sort of nutrient deficiency.
The book "Dogs: The Ultimate Care Guide" suggests adding digestive enzymes to the food to ensure that your dog is thoroughly digesting his food. Pancreatic enzymes may be available through your vet or possibly over the counter.
Make sure that you are feeding a high quality food to ensure that your dog is getting all of the necessary vitamins and nutrients his body requires.
You could try increasing his access to chew toys or bones or something to keep him busy. Extra exercise, play sessions, and close supervision may all help to keep his nasty behaviour under control if he is doing this because he is bored.
If this is purely a behavioural problem, you need to continue to try your best to remove all access to feces. Most dogs enjoy seeking out and consuming cat feces possibly because of the high protein composition found in cat food. For this reason, you have to make sure that only your cat has access to the litter box.
One of my Giant Schnauzers makes special trips downstairs to see if one of her feline brothers has left a treat for her outside of the box. A disgusting and frustrating habit indeed!!
Hope this helps,
Sincerely Zena & Zippy