I know how hard it is to keep going. I screwed up this weekend. I think the main thing you have to remember that is if you give an ultimatum you have to be ready to accept his decision if he decides to end it. He might not be ready to decide yet which of course could cause him to choose differently then what you might want.
It's hard I know, I'm about ready to give an ultimatum as well, but am trying to hold off a little longer. I kind of feel that my wife is holding off deciding about what she wants to do possibly in hopes of me doing the dirty work for her and ending it that way. Then I'll look like the one who wanted it. I know that you shouldn't assume though and give them their space until the proper time comes. When that time is sometimes seems like the million dollar question.