Won't Go Legal

I agree with mtnvly. You might also send him a confirming letter. It should state that; on date & time you said that you would take a lower paying job and fight for custody of the kids if I filed for divorce. Close the letter with this sentence, "There is no need to respond to this letter unless you substantially disagree with its contents."

Find yourself a lawyer and go hose that bad dog.

If he takes a lower paying job now , the courts will see that as such and he will be accountable. Just make sure that you have a good lawyer and that he is aware of this. When your child support etc is figured they should take into account the last few years to show what kind of income he is capable of earning.
My boyfirends soon to be ex is going to have to show the last 7 years of income tax returns. Because she  has a college degree they consider her capable of earning more than an avarage income. She will be accountable for what she is capable of.
From everything I have read, if you can prove that he only took a lower paying job so that he had to pay you less then you will be ok. They are not allowed to do that.
Also , I know of a situation where the wife is getting more money because she gave up a career for her husband ...that may be of help to you.
Take some time for you. You have to take care of yourself for the kids....
Good luck.