It sounds like a good conversation, all in all.
He's right -- changing because someone else needs you to change in order to stay married to you is no way to live. I wouldn't want to. I'd want out, too.
But of course, you're not primarily wanting him to get sober for your sake, but for his own. It will probably be good for him to not have you to blame for trying to sober him up, you know? When there's nobody to fight, no railing that says "do not go here," he will have to find out for himself what's on the other side of that railing -- whether it's the green green grass of being able to drink to his heart's content, or whether it's a sharp drop off down a cliff?
You're smart to have your lawyer lined up. With a kid with special needs, disrupting his education the last two years of high school sounds like a plan I'd like to question. Also, your AH's attempts at controlling what happens also sound like he's in no way ready to let go. I would expect pushback on everything including the agreed-upon separation, honestly. So having a good lawyer on your side will be good -- and then let's hope you won't need her services.