My experience. I "took a break" from the relationship when I was dating my first husband, who was a raging alcoholic (not rageful toward me, just a raging drunk). In the couple months we were apart, he went to AA (at age 21) and in the ensuing 30 years he has not picked up another drink. He was sober for a year when we got married. We split up for other reasons.
I married my second husband (yup, I can pick 'em) during a sober interval after he had almost died from alcoholism (liver failure, whole nine yards). He had been mostly sober with a couple of slips for several months when we got married. We moved across the country, and he promptly went back to drinking, lost his job, would not look for another, and spent his days drinking or passed out while I was working. I moved out after about six months of this, divorced him, and as far as I know he is still drinking himself to death fifteen years later.
So maybe yours will get well, or maybe he won't, but what you do has relatively little to do with it. You have to look out for your own happiness in life, and so far, he seems unwilling to give it up. His complaining about his family not "choosing the right rehab" for him is classic blame-shifting and excuse-making.
Cancelling the wedding is a good beginning. I really suggest Al-Anon--it was a lifesaver for me--helped me to calm down, think straight, and make good decisions for myself.