J. B said: "Nothing seems to ease the fear... So if we try to go off the Miralax while he is in this state he's definately going to be constipated again in no time from holding it."
Well, the problem is that the Mirilax probably isn't actually fixing the real cause of the constipation!!!
Say a person keeps hitting their thumb over and over w/ a hammer - the thumb is going to hurt. You can take Advil to try to make the thumb hurt less, and when you stop taking the Advil the thumb will hurt again, but the cause of the pain is the HAMMER and *not* the Advil. Just like your scenario - the cause of the constipation is still a mystery, and the Mirilax is like the Advil trying to help temporarily, but you still need to figure out what is CAUSING the constipation. Does that make sense?
Please know that it is not, NOT normal for a person (child OR adult) to have to be constantly on Mirilax to avoid constipation. Diet is one of the biggest causes of pooping problems! Have you kept an ongoing list of the foods your son eats every day (called a "food diary") to see what his eating habits look like? Is he eating lots of cereals, breads, cheeses, yogurts, and other cow's milk products -- and not as many green veggies, red veggies, fruits, etc? Because that'd be a big, red flag right there.
Have you tried changing his diet yet, as several moms suggested to you previously? If you haven't tried changing his diet yet, please consider doing this ASAP. Just as a reminder, one very typical cause of constipation in young kids is cow's milk -- so removing all cow's milk products for 3 weeks to see if his constipation disappears would be a good place to start. If his constipation goes away, then clearly you've found the cause of the problem - so simply avoiding or greatly limiting his cow's milk intake would solve the problem, and there'd be no need for Mirilax.
Anyway... just another mom's perspective. I don't personally like to take unnecessary medicines, and I don't want my kids on them either -- especially when the problem still exists (for example, hammer hitting the thumb, or cow's milk blocking up my kid's intestines) and the medicine is just addressing the symptoms (thumb pain, or constipation) and NOT really fixing the actual problem.
Whatever happens, I hope your little guy is able to come to a point where he can poop comfortably and without fear. Your patience and love and awareness of the causes of his constipation will help him get there! Best of luck to you!