J.R.
C.,
First, I think the frequent use of Miralax is compounding his problem. laxatives can cause dependence, and it's possible that you are medicating him to the point where he is not capable of going on his own, without the assitance of a laxative.
If I were you, at this juncture, I would bring him to a gastroenterologist and a nutritionist to figure out if you can retrain his body to go on it's own without the use of laxatives.
Miralax also can take up to 4 days to really work, it's not a fast acting laxative.
Seriously, I would stop the laxatives - today - and figure out what is causing his chronic constipation with the help of some professionals. And, most importantly, if your pediatrician actually recommended that you use Miralax as a long term solution, get a new pediatrician. he is far too young to consider his pooping-problem as requiring assistance from OTC drugs... it's not working anymore because his body has become used to the dose, and now it needs more to do the job. That's how addictions suck you in, and right now, it sounds to me like his bowels are addicted to laxatives.
Good luck... I hope he finds relief.