Six year old poops his pants when upset and angry

I am working with a child that seems to be using pooping in his pants to get attention or control (when upset).  The parent is not supportive and often shames him, makes him wear diapers, and clean himself.

He also flies into rages (tantrums, bangs head) when he cannot accomplish something or during transitions.  He takes medication for ADHD and mood stabilizers.

He has been shuffled around a lot, and I really want to help make a difference for him.  Any thoughts or suggestions will help.  I feel like the poop in his pants issues give him some level of control, and I want to find a way to shift his self-control to something more positive.  The parent will NOT be supportive at all.

Thanks!