Wait for his message and if he says to meet him somewhere. Ask him what time and where. I would arrive early so that you can get a good seat near the exit.
I think you should listen to what he has to say. You can try negotiating, but remember...he's looking out for his best interests and not the best interests of you and your sons. This why he's wanting to do this outside of the court...he doesn't want to have to spend his money on court fees nor be "forced" into paying child support at a certain time, nor be ordered to pay delinquent child support that is owed to you for your sons.
Once the discussion comes to an end, advise him that you need to think about what has be discussed and you'll get back to him. You see, I have seen this type of thing go down many times and it doesn't always end well. Why? Because you can't trust the MLCer. Today, he's willing to tell you what you want to hear and tomorrow he'll swear that is not what he said or agreed to. It's he said, she said conversation and if you don't have someone there as a witness to the conversation, then you have no proof that he agreed to anything you negotiated. Do you see where I'm going w/this? You would need to have a document typed up with your negotiated items/terms and the both of you would need to have it signed and notarized because I do not trust this man one bit. If he's not in agreement with having the document types, signed and notarized so that you can give it to lawyers, then the negotiation would be off the table...but you need to keep your lawyer in the loop and see what she advises you what to do. Do not agree or finalize any negotiations w/o her input. Okay?
No, you don't need to vomit! You can do this. You just have to keep your anger and emotions in check. You have to be on your toes and two steps ahead of him.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.