You know, when it comes to elimination, we always choose to believe the child KNOWS when to go and has complete and full control over it. This is NOT always the case.
Has he had a complete physical exam AND has he had an evaluation with a child psychiatrist (the medical doctor, not the therapist) for THIS??
If a child is in an abusive situation or has been sexually abused, this is common.
If the child has a colon, anal, or rectal problem, this is common. (think Pain when releasing will cause a child to avoid bowel movements)
Also, remember, in a child's life, there is only one or two things they have control of; what they put in their mouths and how they eliminate it. AND, let's face it, the eliminating is the one they have the MOST power over. This is an area where pushing you're own will proves futile and frustrating. Stop with the punishments and focus on the other good things he does...this is becoming TOO much of the focal point in their relationship with him--their whole world is all about his bowels.
So, if they're going to be upset about it; take him to a good CHILD PSYCHIATRIST for a thorough evaluation, take him to get a complete physical exam to rule out structural or functional difficulties, and allow him the pleasure of cleaning up after himself--stop making it an issue and HE'LL stop making it an issue whether it's in his control or not.
Avoid assuming anything.