Hi K.---This can be a huge problem because now his colon does not know how to function properly. Miralax, or any other type of medical intervention, should only be used temporarily to get the body back to where it is operating normally on it's own.
Having said this, the first question you need to ask is what does his diet consist of? To know for sure, please keep a food diary. His diet, and the rest of the family's for that matter, should consist primarily of fresh fruits and veggies, whole grains, legumes/beans and occasionally nuts and seeds (this contain lots of fat and should be consumed sparingly). Animal protein, and that includes dairy, again, should be consumed sparingly. Meat and dairy do not move through the digestive system easily as it has no fiber in it. Dairy can be one of the most constipating substances on the face of the earth. Please go to www.pcrm.org to learn more about the benefits of a plant-based diet. Put dairy in the search box.
I have some other information that I could share with you, if you'd like. I am taking wellness classes, taught by a Naturopath who has her PhD in Nutrition. Unfortuately, traditional medicine/Doctors don't know everything and they especially don't know the medicinal and healing properties of food. It is actually recommended that, if eating a proper plant based diet, that we would be pooping after every meal. When waste remains in the intestines for too long a time, toxins, excess hormones/cholesterol, etc, all end up being reabsorbed into to bloodstream to some degree.
I think if you get him eating more plants/fiber, he will start pooping. He won't be able to help himself AND eating those plants will make it much easier to poop as well. If he is a picky eater, I have some suggestions for that as well. Good luck! I look forward to your personal message. D.