My daughter peed on the potty once a day. When I realized I only had 4 packs of diapers left (I buy in bulk) I decided I didn't want to buy them anymore! So I took her t the store and explained to her that she was not a baby anymore and that I wanted her to wear big girl undies like mama. So she picked them out and I basically just pushed her into it. I never gave her the opportunity to poop anywhere else. When she did the potty thing only once or twice a day, it was always pee.. never poop!
So the first 3 days of her being in undies I just reminded her every 20 mins or so that she doesn't wear diapers anymore and has to go on the potty. She didn't poop the whole first day or second, maybe she was scared but on the 3rd morning that was the first thing she did!
Don't give him the option of pooping anywhere else. If he does good on the potty while he still in diapers, he'll probably be okay to jump into it.
at 12:33 AM on Jun. 5, 2009