Listen. Validate. You don't have to agree. Make him feel like you hear him. Repeating their words w/o agreeing can make them feel like you are really listening.
When my H dropped the D bomb (once & only once a couple of weeks ago) it was SOOO very hard. I cried & fell apart and told him that it wasn't what I wanted for us and for our family. But, I did not negate what he said or his feelings. In fact I told him I did want him to be happy (which is true, but not w OW--those pesky conditions are still hanging on which is a sign of NOT being detached).
It is so hard to do this and detach from your own emotions. I know it is for me. Don't initiate R talk. Let him.
My H also brought up the whole "mediator" mention. I just said, "I haven't put ANY thought into how a D would be." My view is "yeah, I'll agree to a mediator IF you agree to pretty much EVERYTHING I want."
Everyone is different--don't spend too much time thinking about this yet. Take one day at a time, will!!! Hang in there.
M- 18 T-21 S-14,11 & 10 BD 6-18-2012 (OW-EA) H moved out 11-3-2012 10-5-13 Me- I want a divorce. I want to move on w my life. 11-25-13 Jointly filed.