DS was the same until just before he turned 4. He sorted it in his own time. He knew what was happening/about to happen and if he was refused a nappy he just did it in his pants. Can't remember what worked, think it was a combination of him having to admire DH and I pooing on the loo and being made to empty the poo from his pants/nappy into the loo, because "that's where poo lives". Time works out many "problems".