Sometimes they have set backs. They never poop as a matter of rebellion or spite. I would suggest taking him out on leash more frequently and praising him to high heaven when he poops outside. If dogs poop in the house it is common that they will poop in the exact same spot every time. Often it is right by the door. In my case it is near a bay window where I have lots of plants. (I guess it is the closest thing to outside in my house)