If your child hasn't had a bowel movement in over a week, the first thing you should do is call your child's doctor. It's not uncommon, and not necessarily dangerous, but your doctor knows your child best and can therefore give the best advice.
You and your doctor can decided the best way to help your son have a bowel movement soon. After that, it will be important to get him on a regimen to help prevent this from happening again. Polyethylene glycol, the ingredient in Miralax and its generic equivalents, can be very helpful, but it can take a few days to work. It's most useful if given regularly.
The key to managing constipation is to make the remedy -- polyethylene glycol, a fiber supplement or another medication -- part of your normal routine for weeks or even months.
Diet is key for managing constipation, too. Make sure your son drinks plenty of water and eats five servings of fruits and vegetables every day. That's not only good for preventing constipation—it's good for your child's overall health.
Keep in close touch with your doctor, so that if there's a little problem you can fix it before it becomes a big one!