Dear Tweeter,
If it turns out his colon is normal size, then perhaps the reason for large stool has to do with health habits. You said in your original post that your son goes every day now. While adults are sometimes different, young people should normally go every day. If he hasn't in the past, then he's probably constipated. If the stool gets big and hurt to pass for him, you can get him some KY jelly to put on there with a damp kleenex.
Constipation can come from a few easy things to correct: Not enough fiber, not enough water, not enough exercise. Constipation can cause the stool to get sluggish and build up to a larger size and so it's hard to pass. For fiber, kids like to eat ordinary Cheerios, use 100 percent whole wheat for sandwiches, and they love spagetti (it soaks up water from cooking). For water, every time he finishes brushing his teeth advise him to then drink a whole glass of water as a habit. Hopefully he has outdoor activities where he runs and plays a lot...some kids get fixated on video games or TV. So, they need to get up and go play catch or basketball with you after school, or a couple times a week an involved sport like swimming or fishing, or even easy walks with you a little after dinner is good.
Probably too much information, but I am familiar with constipation and wanted to pass along everything, in case there is an item or two that might help your boy.
GG
Son's doc said two weeks - seems too short of a time for "something" to return to it's normal size after probably years of being "enlarged". She suggested a specialist but seems a bit early in the scheme of things for that.
I'd suggest you ask the doc who suggested the MiraLax just how long your son should remain on the product. Two weeks is a fairly short period of time.
Part of the size of the BM's may be due to the actual 'diameter' size of portions of his colon, so has the doc suggested doing any testing like a barium enema or other tests?