I would always remind my son that he was wearing undies. and show him how it feels different from wearing nappie compared with undies. I used to poke my little ones bum and go! Oh your wearing undies, and ask if he wanted to sit on the toilet / potty.
Hope it all works out for you. (pull ups helped alot, as my son didnt like it when the picture would fade away)
GOOD LUCK!!
~ Yep it's Jay