My 18 month infant has constipation and when he needs to go out he is in pain as he cries and runs around holding his butt. what will help?

Stool retention: This is a sad problem that is all too common. After a kid learns s/he can hold it, they do so at some point & when they do go it is bulky or dry & it hurts to pass. They then hold it to avoid further pain & the cycle repeats. They don't understand they could avoid pain if they didn't let it bulk up. This often requires a stool softener like Miralax used daily for 2X the duration of the problem.