What about trying to put his undes on the outside of his nappies or pull ups? That way he might still feel like he's in nappies, but it looks like he's in undies - this may do a alot for his self esteem in relation to this.

I don't mean to pry, but is there a reason why he is afraid to poo in his undies? Has someone yelled at him before for having an accident in his undies? (Not saying it's you, but might have been at child care?) Perhaps thats something you need to look at as well.

Good luck!!