Let him do whatever he'll do. You have no control over that anyhow. You can however work to provide a stable environment for your son, (being surrounded by extended family is a good start), and help him work through the transition. It won't be easy, obviously, so that's where face to face (and online!) support come into play. I found a lot of strength in an online group of single mamas all going through divorces and separations.
Eventually, once your ABF realizes that you refuse to be his enabler, he'll find someone else to mother him...or he'll hit rock bottom and then, who knows? Whatever the case may be, give him the opportunity to go on that journey so that you can go on your own.