I think you have to be careful before you leap to conclusions about any condition at this stage. Lots of kids have constipation (v.bad at that) and the important thing right now is to just keep him regular. Plenty of fluids is the key, and I'm sure I don't have to lecture you on fruit&veg!
I have to monitor my son just the same, & ensure he's drinking enough (dehydration is not just what happens in the foreign legion) - many's the time I worry about early onset and I do empathise but equally it's important to maintain a sense of perspective and just keep them eating/drinking healthily.
Keep up with the Miralax and suppositories. Clearing the constipation is important. Severe constipation was one of my son's first complaints as well. If his symptoms persist and you are sure he is no longer constipated, ask for a referral to a pediatric GI specialist to have him evaluated. There are so many possibilities. It took us about a year and a have to finally have a definitive diagnosis. It seemed like we spent more time ruling things "out" than we did confirming what it was. The process was scary and frustrating, but well worth it. Good luck to you both.