In adults, this problem is called fecal incontinence, and is not considered to be the fault of the sufferer. It can be caused by certain bowel problems and/or medications.
I very occasionally have the problem myself, and it always comes as a surprise - not because I'm too busy or lazy to get to the bathroom. One doctor told me my problem is probably the result of physical damage to my pelvic floor from a forceps delivery, but I have known a couple of other men and women who have mentioned the problem who haven't had forceps deliveries. Oh, we seniors have such interesting conversations. ;-)
Please don't assume it's the result of willful or neglectful behavior by your son. It could be, but probably isn't. Do bring it up with a pediatrician, and soon, if it's a frequent problem.