I would go on my attorney's advice. If you know your ex is under reporting you have a legitimate need to protect yourself. My philosophy in general is to not be a part of anything that is illegal or even questionable. There is no way to know ahead of time what the government will do. Better to be safe than sorry, as the saying goes.
As to his persistence I'd assume he is looking out for himself and is not concerned for your well being. I'd tell his attorney that all communication is to be done thru your attorney and then stop worrying about it.
I've not heard of a tax advocate but if there is one I"d talk with them to get information and advice. I've found that nearly everyone that I've talked with at the IRS has been helpful. I've talked to several over the years.