Way too much Miralax! I agree with the other responder(s) who stated that is too much Miralax. I did a capful 3x day (up to the white line...not full) for five days. After the five days of three times a day, I gave it to him once a day for 6 weeks. I put the Miralax in a small bottle of Gatorade, and mixed it until dissolved (anything your son likes to drink will work - except milk as it will gel up in milk). The doctor we saw said this will soften everything, but he won't be scrambling to the toilet with diarrehea. He will have the urge to go but it won't be a explosive liquid nightmare. It slowly moved everything out so he can resume normal function.
I guess constipation is a huge issue for kids this age, this time of year, because they just aren't drinking enough fluids because they aren't hot and sweaty. As long as you do 1 capful a day for weeks instead of numerous capfuls for a short period of time he should be fine. Also, I sent a water bottle to school with him which his teacher allows in class.
Also, I just learned yesterday from a candy vendor that if your child likes licorice...buy the Australian kind as it has anise, which settles the stomach, and acts as a laxative. Also, the Australia brands have no natural coloring or flavoring, if you get the good stuff, so it is a great way to bolster your digestive health.