Welcome back.
First off, I think it's great that you've taken the important step of protecting your son. My youngest is five, and also about to start kindergarten next year, so I know how heartbreaking that is. My son loves his daddy, but his daddy is too dangerous to spend time unsupervised with him.
Whether your wife chooses sobriety or not, have you done anything for yourself? Protecting your son is a great step, but are you protecting yourself? I know you said you've been here before, so forgive me for being unfamiliar with your story. Have you tried any programs like Alanon or celebrate recovery for yourself? I attend Alanon meetings, but from what I understand CR has programs for young children, so you could take your son along if leaving him home with your wife isn't an option.
Trust me, I know all about that shifting line in the sand. "If x happens, I'm outta here." Then x happens, but it doesn't seem so terrible,and leaving is so hard and what if things really do get better this time? That's why Alanon has been a lifesaver for me. Learning about boundaries, unacceptable behavior and my own role in the whole mess.