Well done on being strong and leaving. My heart goes out to you.

I've been sober almost 5 years and it was only when I realised that I was going to lose EVERYTHING that I stuck to actually giving up the drink.
I had done it a few times before but when it seemed like it was forgotten I just started drinking again. (so as you said 'will he assume that if he messes up again it wont be a big deal" - that's what I saw it as.)
Stick to your guns, give him support if needed, giving up drinking is hard but don't give in.
Time frame wise I'm not sure, maybe he has to join AA or something to prove that he is really commited to you and not the drink.

Hope this helps in someway xx