Miralax is not a laxative, in spite of the name. It's a compound that is not digested, but passes all the way through the intenstines along with fecal matter and helps keep it moist and softer. I've discovered from personal experimentation that Miralax can cause diarrhea, however, so I'd introduce it in gradually increasing doses, and for most kids, yes, it does make withholding more difficult, if not impossible.
I'm wondering whether your son has felt pressured to use the toilet. If this happens (particularly if the pressure/manipulation started before a child is physically able to be trained), many children become reactive and defensive, and a withholding pattern starts. This can result in large, painful-to-pass stools, and then fear makes the child even more likely to withhold. They're too young yet to "get" on an understanding level that they will suffer more if they withhold. They just know they don't want to suffer right now.
My grandson was/is very prone to getting constipated, so this has been a bit of a struggle at times for him. His parents increased the veggie and fruit content of his diet, encouraged him to drink as much water as they could coax into him (I have "drinking races" with him every time I spend the day, and he'll down a whole glass of water to win), and very occasionally helped him when he was "stuck" with a children's glycerine suppository. He hated those, but was always so grateful after he pooped. The addition of Benefiber and occasional dried fruit WITH lots of water seems to be helpful.
I'm glad you're taking action early. A difficult condition called encopresis can quickly result from children withholding or a bout of constipation. Then they really lose touch with the urge to have bowel movements, and accidents ensue. Google encopresis for lots of information.
If your son is expressing fear, there are acouple of books some moms have found helpful: It Hurts When I Poop, and Everybody Poops. Also, the link GrammaRocks gave you is a good one!