3yr old in pain..

Cant poop .. he crosses his leg when he needs to go but it wont come out I've tried Miralax what can I do ease the pain 😭😩.. it's been going on for 3 days now (er said to give him Miralax but it's not helping he is soooo picky he wont eat fiber I'm trying so hard what should I doo to help him go.. hes drinking lots of water