I agree you need to have a good plan for if he doesn't agree to end the A OR to move out...this is the high likelihood. I think you can say that you wanted to have this conversation first, and that your follow up will be to meet with an attorney to establish your rights and take the appropriate next steps. You don't have to say the words "file" or "divorce", and if he challenges you and puts those words in your mouth you can say something like:
"I don't know what those next steps will be, that's why I will be getting professional guidance. My only goal is to protect myself from the choices you've made."
Without coming out and saying it you're setting the tone that you're not retaliating or punishing, you're not filing or trying to rake him over the coals...but instead of saying that you're simply saying that you're getting professional council on your rights and the appropriate steps given the situation.
Rehearse rehearse rehearse!
You may also want to practice a line to end the conversation. I have found that sometimes we stick to our script for a while, but if they keep going over the same ground again and again it can leave us feeling like we need to explain more, defend ourselves, expose ourselves, etc. Don't do that. There is nothing more to discuss. Find a friendly way of ending the conversation.
"I have already covered where I am at, what I'm doing, and why. I don't expect you to agree with my decisions as if it was possible for you to understand the impact of your choices then you wouldn't have made them. If you have something else you wish to tell me I'll listen, however if you don't intend to end the A or move out voluntarily then there really isn't more to discuss at the moment."
You can do it!
Me:38 XW:38 T:11 years M:8 years Kids: S14, D11, D7 BD/Move out day: 6/17/14, D final Dec 15
