The thing is, I don't see this as "choosing" like with a child who does it sometimes, but other times is just too busy or lazy and doesn't want to take the time to go to the toilet. It is definitely a comfort thing. My daughter has pooped a tiny bit on the toiet a couple of times too, but I know she is just not comfortable...she can't relax enough to let it come out. I have tried the potty as well with no success. I would really try to avoid making it a power struggle. If he goes to bed in underwear, will he poop in those too (I know my daughter will, but maybe your dcb is further along in the process)? Could you make it really fun to wear underwear to bed? Like some special underwear that he gets to decorate (get his parents to supply them) with markers or fabric paint and become his magic naptime underwear, but he can only wear them if he first poops in the toilet or potty ? Personally, I would not punish him for pooping in his diaper as I wouldn't want to attach shame to potty training in any way.