Harper...the general standard that has been used is to look for one year (at least) of uninterrupted sobriety....
Also, that gives you time to attend alanon and do your own work on yourself.
His best chance of gaining true sobriety...and, not just white knuckling for a period of time....is to get into a program....AA, sponsor, counselor, etc....and working the program with diligence...as a top priority for himself.
It is through working the steps that the necessary internal changes come about. The changes in alcoholic thinking...which leads to changes in attitude...which leads to changes in behavior.....
It takes persistence and work and time. Most of all, he has to want to get sober.
Rehab can be helpful, in that it points the person in the direction and can give them some to ols to work with,,,But, it is by no means a magic wand.
The recovery can be done through daily AA meetings and the "program".
In fact, he has already been exposed to rehab as well as AA. He already knows what the deal is. He knows where the help is if he really wants to get sober.
Honestly, from what you write, he sounds like he is resistant, at this time.
I think a year's time will begin to tell the story.
Don't make a boundary unless you are willing and able to back it up.
Have you read "Co-Dependent No More?"....it is practically a "bible" in these parts...lol...