Our 20 month old son is potty trained since January. This week he got constipated. We used Pedia-lax to get him to poop. However, the last time we used it (Pedia-lax) he was clearly not constipated anymore. He seems to be afraid to poop again, probably because he is afraid of pain. He repeatedly requests to go use the potty but doesn't poop. He only cries and gets all worked up and refuses to stay on the potty and try to push. How do we get him to poop again normally on the potty?