When you say your "past restrains you" from taking your son, what do you mean? Do you mean it emotionally restrains you (not wanting to take a child from his mother) or is there some kind of legal restraint that prevents you (e.g., a probation condition)?
Either way, you can't simply leave the child in the hands of a raging, irresponsible drunk. If there is some reason you truly can't take care of the child, please call child protective services. They won't necessarily take the child away immediately, but they can open an investigation and provide services and monitoring. If your partner refuses all help, then yes, there's a chance she will lose the child, but the child will be placed in a safer setting.
ETA: You are legally responsible for continuing to provide financial support no matter what you decide.