i would say that a divorce would add a lot of stress and probably exacerbate her drug problem. This does however sounds like child endangerment and you may want to file a police report. It sounds like you are using this as an 'excuse' though; is divorce really in your childrens best interest? or...
it sounds like this poor child is probably feeling a bit unloved at the moment. Is he on medication? Is he a candidate for medication? Its possible that the mood disorder is a chemical imbalance. My life was somewhat out of control until i began medication almost 10 years ago. I feel 'Normal"...
why not..
why not change your last name to your mothers BF's last name, then your child will also have your last name and his? plus you wont have to consumt biodad, i dont think.