I had problems with my son doing this. The reason was: I worked at the day care and the school's policy was that they HAD to be FULLY potty trained. They made the exception because I was there and "could change him" whenever he had his daily poop.
Well, Christmas break came and I was given another class for the 2 weeks because of the fewer numbers during the holiday. I was not in the very next room during that time and it was not as CONVENIENT to change him (I had to get someone to cover the class and go take care of it instead of just having him come to the bathroom off of my classroom). I had also just discovered that I was pregnant with #2 and was determined to get him trained before the baby came.
Well, I made it a completely NEGATIVE experience for him every time it happened during those 2 weeks. It was to "fun" to get to see mommy every time, so I spanked him every time during those 2 weeks. About the 5th time, he realized that I MEANT it and quit.
It sounds as if you are possibly giving him too much attention for the negative behavior. If you do not beleive in spanking, then make him stand in the tub and change himself and then use ONLY cold water to wash with. If you do not speak to him during this except to instruct him on what to do "Take off the underwear" in a mono-tone voice (with NO personal words like, "Honey, take off your dirty underwear.") Then make him sit in time out after he is fully dressed. Beleive me, after a few times of this, he might realize that it is NOT a positive experience.
It's all in the math: (-times-=+)
Negative behavior X Positive reaction = Negative behavior
Negative behavior X Negative reaction = Positive behavior