My son drink soy to when he had that problem i just gave him 1 ounce water 1oz prune juice. He pooped later that night.

A few things you can try. One is the bike motion which is move the baby legs like they are riding a bike (this worked for my daughter 20 mins later she went) or 1 oz apple or prune juice and 1 oz water (it's ok that you give them that I did with my daughter I got the ok from her doctor)