All I can think of is to keep at it. Don't let him poop inside. Has he even once pooped outside? If he does, give him an extra special treat, something he LOVES and praise him a whole lot. Another thing you can try is when you take him outside and you think he should have to go, insert a small, cardboard (not the wooden ones) matchstick half way up his behind so half is still sticking out. Have the sulphur end to the outside. This can stimulate him to go. Walk him around a little first. Make sure he has enough fiber in his diet too. When he does start to go, quietly add in a cue word, like "go poo" or whatever so he'll learn what you mean when you tell him to go. Don't forget to praise him and give him a marvelous treat the second he finishes. Don't make a big fuss while he's going or he might stop. Wait till he just finishes.