I know this has got to be hard for you to go through an him whining, begging an threatening makes it harder. In my opinion you need to stop second guessing yourself. Like the others have said stick boundaries seems he is more concerned with getting his self back in the house, than getting help. If you back down now nothing has changed an mostly things will just go back to the way they were. Drugs or no drugs I have seen it happen before with abusers.
Good luck an remember don't let him push your buttons, an take things one day at a time.