This is a common problem in dogs, and most the time it doesn't mean anything at all. It is just a bad habit. They may find it tasty or they may do it out of boredom.
There are a few situations where this could be a symptom of an illness, but your dog would be showing other symptoms such as weight loss, chronic vomiting or diarrhea, or possibly an increase in water consumption. So, if your dog is okay in all other ways, then chances are it is just a bad habit.
The best way of dealing with the problem is to simply pick up the poop as soon as he goes so he doesn't have the opportunity to eat it. There is also a product called Forbid (you can get it from a vet and probably at pet stores too) that you sprinkle on their food that is supposed to make the poop taste bad to them. Sometimes it works, sometimes it doesn't. You can also use Accent Meat Tenderizer (in the spice isle at the grocery store) sprinkled on the food as a possible remedy. Another option would be to try sprinkling something really awful tasting (most people use tobasco sauce) on the poop as soon as he goes and then let him eat, to try to teach him that poop tastes bad. This seems a bit harsh to me, so I wouldn't really recommend it, but I know that it has also worked for some people.
I hope this information is helpful. If so, please click Accept. If you have additional questions on this topic, please let me know.