[Soundy]

Has this dog been kennel trained at all? If so, might be time to go back to that: if the dog doesn't poop when you go out, put him straight into the kennel when you come back in. Give him an hour or so, then take him straight outside. If he still doesn't poop after a good walk, do it again. His choices then become to either poop outside, or soil his own bed, something dogs REALLY tend to avoid.

Walking will usually help loosen up the bowels too, so make sure you actually take him for a good walk, not just let him out to run around the yard for a bit.