Get the ball rolling. Take a little bit of control and give yourself a project to distract your mind. This is win/win. If/when he does leave, you'll want the income that can produce. He'll also see you taking him seriously and not dismissing his concerns.
This is going to sound like a broken record, but it's good advice: I think right now you need to work on yourself. Really reflect on yourself, make yourself a better person. Pay attention to your family. Detach. It's hard, but it's really the best move. Find some 180s for yourself. For instance, my H complained I was a poor housekeeper. (Jerk!) I'm not a poor housekeeper, but I'm by no means Martha Stewart. So I started making the bed everyday, picking up a little more and actually doing more little things that make a big visual difference. You know what? H didn't expect it, I don't find it to be that big of a deal and my house is actually easier to take care of. It's a positive 180 regardless of how we feel about each other. Go find some things like this.
Basically, you can't change the situation right now, but you can make baby steps for yourself and these babysteps can be great for your relationship.
Me: 42/H: 37 T: 10 years/M: 8 D9, S8 Bomb: 7.23.07 Separated: 1.20.08 D Final 3.19.09 Affair started in '05, found out parts in 11/07. They married 11.26.09