My son very well may have this though his GI has never done a scope. All of his symptoms are similar. We were on Miralax daily forever, but it didn't work that well and gave him painful gas and bloating. His doctor switched him to 64oz of water daily, eat more fiber (which he is NOT doing. He's 15 ), docusate sodium twice daily (which works by forcing more water absorption into the stool), and 1000mg or more of mag a day (with instructions to up it if necessary.) And also a 15-strain probiotic that contains bifidum infantum. It works really well for him now. One day when he is older he may have a scope done if the help stops working, but for now he is functional.

I hope you can take some help from some of that.