The choice is not as clear cut as you make it out to be.
The delusion is that you have the choice to "abandon" him or stay and "help him get better."
The reality is that if you allow him to remain in your life he MAY get better, but the odds are far more likely that he won't, and if he does it will have little to nothing to do with you or anyone else in his life. The choice is to protect yourself and your unborn child by withdrawing or risking dragging yourself down into his mess by staying.