Coprophagia (Eating Feces)
Although most people find it repulsive, it is quite natural for dogs, especially puppies to eat feces. (Feces found in kitty's litter box is an especially favorite treat.) Fresh feces closely resemble the first solid food the puppy ate: warm, semi-solid, semi-digested food that mom regurgitated for her pups to eat.
Often this behavior will just go away when the pup matures. In the mean time, keep the pup's toilet area as clean as possible. Be sure that the puppy is checked for worms and other possible problems such as something lacking in the dog's diet. It is a good idea to have the dog's diet approved by your vet, or switch to a nutritionally complete dog food sold by your vet or pet store.
Try the following to train him to leave the poo alone: