My suggestion would be to walk him at night UNTIL he poos. He's not getting enough time outside to "let her rip" maybe?
Pooing in the house - when genuinely house trained - can also be a medical reason. If the poo is not unusual or you suspect a parasite or something, I'd try the aforementioned route.
Good luck. And where are pictures of this gorgeous boy for us to feast our eyes upon? You cannot get away with posting her without PICS!