Well, I am probably not the best to weigh in here, so I will keep my personal thoughts to myself and advise that you follow your instincts in your dealings with him.
know that at some point it will most likely get nasty. doesn't mean it will remain so.
I don't think you necessarily have to tell him anything. you could say this is a shock and you're trying to get your bearings. stall for time.
M 20+ T25+ S ~15.5 (BD) BD 4/6/15 D 12/23/16
"Someone I loved once gave me A box full of darkness. It took me years to understand, That this too, was a gift." ~ Mary Oliver
But you don;t want HIM to think you're stalling - that will only ramp him up more. So use some emotional aikido and convince him that you are equally eager to get this done. Then you can let your lawyer handle things once you have a plan.
I would gently respond to him and say that you've not been given ample time to review the document and you need to retain a lawyer and allow him/her to review it as well and once that is done, then your lawyer will advise his lawyer of what your concerns are.
You can also state that you do not want any costly mistake to be made which will cost even more in out of pocket expenses. If he gets to be too much, advise him that you will not be bullied into signing anything without legal counsel and he needs to stop threatening you because you are just as eager to get this over and done with as he is...but it's going to be done with proper counsel reviewing it and advising both parties as to what is acceptable.
Besides, no one wants to sign a document that is just plain childish and irrational and being bullied to do so. That is attempting to get you to sign a document under duress and that isn't something anyone wants to sign. His lawyer knows this...I wonder if his lawyer thinks you are as irrational as he is.
All of his posturing makes me think he's hiding something and doesn't want you to know about it. I swear, I would have to point blank ask him what is the rush and who is she just to see what his reaction would be.
If he gets to be too much, take the boys and go somewhere else tonight. You do not have to listen to that BS and I would not respond to any more of his text/phone or email messages for now. The more you respond, the more he's going to ramp up. He thinks he's got control over the situation. He doesn't....you do, so take your time w/it and get a good lawyer.