When DS switched to whole milk from formula we had some major constipation issues. We also figured he was also being a poop holder -- associating stooling with discomfort/pain, so he'd hold it in. Create a vicious cycle.
We were able to deal with his constipation issues with Milk of Mag because thank goodness it was short lived. But I do recall his doctor going over the different meds used to handle constipation and blockages -- miralax, mucomyst was one, go lytely for more extreme circumstances... And one thing he mentioned was that one might be tempted to decrease enzymes, but that can sometimes cause problems in the long run. Same sticky mucus that can cause meconium illeus in newborns can also cause something called a meconium equivilant -- stool gets backed up into the colon, yet you can still be stooling.
I do know a friend of mine's granddaughter is on miralax on a regular basis to keep things moving along.