I know what you're going through. Harley had the poo problem when he was really little but I discovered the cure (for him atleast)
After he goes pee pee get him to run around a lot to get him moving. I started taking Harleys fav toy outside and tossing it about for him to chase and run around w/ and after he got going 'boom!' it hit him and he realized wow, I need to poo! Afterwards make sure to reward and keep it up. Don't bring him in until he's done his deed. lol That might sound dumb but it worked for me so I would atleast try it!