Ok, my son is 3.
If your ds poops in his undies, have HIM clean up the mess. He needs to put the poop in the toilet and wash out the undies by hand in the sink (you can help, but he needs to do enough of the work to be affected by the consequence of his choice to poop in his undies). Right now your ds has no motivation to poop anywhere else because HE is not being affected by his choice since you're the one doing the icky work.
If washing out his own soiled undies isn't motivation enough, you can add additional negative consequences, such as loss of TV time. Pooping in undies needs to become very undesirable to ds.
If your ds asks for a pull-up or uses the toilet, he can be rewarded (small reward for pull-up, and bigger reward for toilet since that's where you really want him to poop).
Thank you for your post......however the idea of my 3 year helping me clean up the poop is less than desirable......ewwww
But, you are right, maybe if he had the consequences it would help.....but since I am an
mom2drewby, Hi and welcome to the Toilet Teaching board.
I agree with the previous poster. Have your DS help with the clean up after an accident. Have him rinse out the clothes, and then put them in the hamper/washer/where ever you would like them, and then have him get himself dressed again. Please keep us updated on your DS progress.