If your son is getting uncomfortable, or if this has gone on for more than a day, you could give him a glycerin suppository (a liquid children's form is available). This usually gets very fast relief, sometimes within minutes.
But you should NOT rely on the suppositories, which should be used only once in awhile at most. Instead, be sure he drinks plenty of water every day, and eats plenty of fiber (veggies and fruit). Sometimes adding some yogurt with "live cultures," or some other form of probiotic, is very helpful.