Hi Ella, maybe look into the future, say 5 years, with the assumption he won't get help for his addiction. Where will you be emotionally and financially? Ultimatums about him seeking treatment can be valid if you have every intention of following through. I know that concept isn't popular in SR, but it would have the advantage of you knowing you had given reconciliation a fair chance.
You don't say whether you have the ability to support yourself and your son, assuming your AH is spending all his income on drugs. From a practical POV do you know if AH is running up debt? You probably realise that if he had crashed the car you would have been half responsible for any liability, and insurance doesn't pay if the driver is drunk.
I do get what you're saying about your happiness not being the only consideration in whether you end the marriage, but the other would have to be your son. Will he suffer from the dynamics in the household?
You need to clarify your thoughts, so why not write pros and cons lists? You may not get an answer right away, but it will bring to mind the practical and emotional issues you face.