Hey I had the same problem with my son, he also started weeing his undies again, I ended up letting him run around in no undies and he just started going to the potty fo poo's and pees. I think it was because he knew he had soemthing there to catch it when he was wearing undies iykwim he is the same as faeriegirls boy, once he did poos in the potty he came and got me to show me whathe had done (very happily) now he goes all the time even when wearing undies. Don't know if that will help. Hope it all works out for you soon I know how gross it is cleaning yucky undies everyday.