Take your son to the pediatrician. It's not urgent, but you need to have your dr on board with this. It sounds like encopresis and its not his fault.
Encopresis is a problem that children can develop due to chronic (long-term) constipation. With constipation, children have fewer bowel movements than normal, and the bowel movements they do have can be hard, dry and difficult to pass. Once a child becomes constipated, a vicious cycle can develop. The child may avoid using the bathroom to avoid discomfort. Stool can become impacted (packed into the rectum and large intestine) and unable to move forward. The rectum and intestine become enlarged due to the hard, impacted stool. Eventually, the rectum and intestine have problems sensing the presence of stool, and the anal sphincter (the muscle at the end of the digestive tract that helps hold stool in) loses its strength. Liquid stool can start to leak around the hard, dry, impacted stool, soiling a child's clothing.
It can be from many things:
-Eating a high-fat, high-sugar, "junk-food" diet.
-Drinking mainly soft drinks and sugared drinks, and not drinking enough water and 100 percent fruit juices.
-Lack of exercise.
-Reluctance to use public bathrooms.
-Stress in the family, with friends or at school.
-Being too busy playing to take time to use the bathroom.
-Change in bathroom routine, such as when a child starts a new school year and bathroom breaks are less frequent than they were over the summer.
Encopresis can cause both physical and emotional problems. Impacted (backed up) stool in the intestine can cause abdominal pain, as well as loss of appetite. Some children develop bladder infections because they avoid urinating as well as passing bowel movements. Other health problems may cause chronic constipation, including diabetes, hypothyroidism, Hirschsprung's Disease, and inflammatory bowel disease. Children with encopresis can certainly feel emotionally upset by the "accidents" they have when they soil their clothes. Their self-esteem and interactions with other people can be affected. Children are often ashamed or embarrassed. They may avoid going to school, playing with friends, or spending the night away from home. Parents may feel guilt, shame, anger or distaste by the problem. The child will often be aware of a parent's feelings and become even more emotionally affected.
My information comes from Children's Hospital of Philadelphia. http://www.chop.edu/consumer/your_child/condition_section...#
There is much more, valuable information there that would just take up too much space to put here.
Good luck!