Dear WLW,
It does sound like ADHD is not a primary concern with your son, and that some additional diagnostic workup is in order. You may find it helpful to have him seen by a developmental pediatrician and by a pediatric mental health clinician as well. The encopresis is probably not an emotionally-based symptom, but does require a systematic management program. All this can be addressed via a developmental evaluation.
I,too have an 8 year old with many problems. His started when he was about 3 and I moved in with someone that he obviously did not approve of. This just set off the classic symptoms of Bi-polar disorder. He also has difficulty going to the bathroom even if only 2 feet away. It does take a systematic approach but also alot of patience and a lot of controlling stressing events.