http://www.emedicinehealth.com/encopresis/article_em.htm is a good reference.
Listen to your doctor, and read up on encopresis in the meantime. (Benefiber is the stool softener of choice around our house.) If your son's lower intestine or colon is distended and not working correctly, he could need this for a year to get everything back to normal function. And the sooner the better, you don't want it to be a problem for him any longer than it has to.