My question is why is my son getting backed up & what could be the possible cause of his bowels not flushing out after 2 round of 64oz of water/miralax at 2 different times. we have done, several xrays and ultrasound. currently waiting for endoscopy?

These are most often products of long term (years) stool withholding that balloons the large intestines, weakening their emptying efficiency. Simply clearing the oversized backlog without long term assistance does not allow the intestine to shrink back to normal size/function. Seek out a Pedi-GI or a Developmental Pediatrician to create a bowel rehab plan. Can take a year or more.

Constipation is a rather common problem, even in young teenagers. It is not unheard of to not respond to MiraLAX or to any other particular medication at times. Sounds like you are properly seeking medical attention from a gastroenterologist if you have an endoscopy planned. In the long run, fiber and hydration can help, but follow what your G.I. doctor recommends. Good luck.