I should add that I'm a pediatrician, so this is my thing. In my almost 20 years of practice I've never encountered deliberate soiling. Almost all cases of encopresis are due to constipation with a very small group caused by anxiety/emotional distress.

You need to be sure your child isn't constipated before doing anything else. A simple x-ray will show this as constipation severe enough to cause encopresis is obvious on x-ray.