It is not unusual for an ADHD child to have these problems. Couple of questions so I can help.
1. Is he on meds? If so, what?
2. Is there a time of the day when he seems to have the most problems? ie. Can you tell if the pooping happens more in the morning or afternoon.
3. Does he have this problem on weekends?
4. Did he have this problem over the summer?
5. Did he have this porblem last year?
The biting an chewing on things is not unusual and I will give you ideas for that - but lets deal with the pooping, peeing first. Oh, has his doctor given you any ideas on things to do?
Sorry you haven't gotten back to me as I could give more specific answers if I knew more. At least check out this link. You should find it helpful.