Some dogs just do this. it could be that he is missing something in his diet that his body needs. I read some were that if you put pineapple juice in thier food, it makes the waste unediable, someone also suggested meat tenderizer.
It's called coprophagy. It's actually more common than you think. I would just add meat tenderizer in his food. You can also purchase Forbid. It comes in pill form and powder form. You can add the powder to the dog food. It gives the feces a weird taste and the dog won't eat it.
My dogs don't go after their own waste, but my older one used to go "litterbox fishing" all the time when I had a cat. It was so gross; if I left the bathroom door (where the litterbox was) open for one second all the poo would disappear! Gross!
I hear that stuff that they sell at PetSmart, etc. that makes their feces tast bad really works!