I think what you've got here is spot-on PERFECT. I wouldn't change a word of it.
Furthermore, I would not feel obligated to address filing one way or another. If he asks you "Do you want a divorce?" just say "No, but I'm no longer willing to live with a third person in our marriage." If he asks you what you're going to DO about it, just say "I'm not sure yet. Right now, I'm just letting you know what my boundaries are, and telling you that the deceit has to stop, because it's incredibly disrespectful to me and our marriage."
Lather, rinse, repeat.
Whatever you do, do NOT reveal ANY of your sources of intel. If he asks you how you know, just say "I'm not going to say -- but I do know," or some such. If he asks you why you won't tell him, just say "Because I don't want to."
Try not to come across as angry or vindictive. Be matter-of-fact, and if at any point you tear up and become upset, that's OK too, so long as you can get it back together. You can tell him you love him, but only do it ONCE if you can.
Really, the tone and content you've struck above is perfect. "Less is more" when it comes to these things. You're better off repeating the above two or three times over than you are expanding upon it.
Finally, if he asks you something you can't answer, just say "I'm not prepared to answer that right now," or even "I'm not sure.
Make sure to be the one to end the conversation. The whole thing shouldn't go on for more than 20 minutes or so.
