If you are not ready to talk about anything legal (divorce, separation) then simply tell your H :H, at this time I am not prepared to discuss anything further about a divorce.
Your H has had a long time to emotionally detach from the situation before he said a thing to you about wanting out. He is in a much different place than you and certainly should take all the time you need to "catch up".
I would not make myself so available to him and I would no longer offer to do him favors. Be very busy and create mystery. If he calls/texts/e-mails wait at least 24-48 hours to respond and when you do respond be vague, happy and upbeat. Let him wonder what you are up to. This will also give you some space from having to talk about something you are not yet prepared to discuss.
Setting healthy boundaries is very important. Working on you (you said shooting your mouth off was an issue for him) is all you can do. Validate his feelings even if you don't agree with what he is saying and do not participate in any sort of R talks, it's way too soon.
Are you 110% certain he is not having an affair of any kind? Between the divorce support group I have been a member of for almost two years, this site, my counselor and "real life" experiences when a WAS moves out and a week later begins talking divorce there is a reason they want out so fast.
If he is going to file then he is going to file. You are not obligated to "work with him" or HELP HIM get a divorce.
