Have your son start supplementing with Magnesium. It will move his bowels and it will not work like a laxative. Have him start with two capsules of 400 mg of magnesium oxide every night before bed. He should start going to the bathroom daily after one or two days. If he doesn't, up the amount of magnesium to three capsules before bed.
He will go to the bathroom upon waking or within a few hours once he's been on the mag for a few days. I have been using magnesium like this for over 10 years. I am totally regular and everything is fine. And it doesn't matter what I eat, for some people it doesn't matter. I believe my slow system is because I have a sluggish liver.
I highly recommend Twinlabs Magnesium Capsules, but you can buy the Phillips Milk of Magnesium chewables now and they work fine. But he'll have to take a lot of them to start, about 6-8. He may find that 400 mg is fine after he's on a regular schedule with it.