Has anyone tried to use a rectal thermometer to help with baby constipation?

My son is 4 months old and has constipation, his poop is pretty much pebbled and hard all the time but he usually is able to poop once a day however every once in a while he'll miss a day and that affects him eating wise(he doesn't eat hardly and pukes if he's backed up). With it being a weekend I am going to try karo syrup since prune juice seems to be harsh on his stomach and takes a while, but until I get a chance to go to the store I had heard that inserting a rectal thermometer will help him poop. I have never done this, so how long do I do it for and how far in , and does he poop instantly or what?