No medical reason that i know of, when he first came to me last fall, he would pee in the potty about once a week if i was lucky then he just stopped. He does not say anything when is wet or poopy. We tried wearing underwear under the pullup but he wet it every time and after a week Mom said it was too much work washing 5 or 6 pairs of underwear every night so we stopped that. He does stay dry during nap but will not pee anywhere but the pull up, I have had him sit for 20 minutes and nothing then as soon as the pull up is on he pees. We have tried getting him to stand, use the big toilet instead of the potty and nothing. Dad apparently has had a couple of successes but I think it was more good luck and timing. It is frustrating because if I ask him if he peed in the potty he will say yes even though he hasn't done anything and when he poops he won't say anything until I ask him. So I don't know what more to do.