He could very well be eating his stool. Sometimes that's a sign of a deficiency of digestive enzymes or just a very bad habit. Take a fresh stool sample to your vet and ask for a 'film digestion test' also have your dog examined by the vet to rule out any other problems.

If that checks out ok, and he's acting fine, eating fine, and playing, then I'd think it was a bad habit and maybe not let him out loose in the yard until you've followed him around to go potty, then pick it up right away so he doesn't eat it.

Good luck