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I'm sorry to hear about your situation. The primary issue that arises is the fact that you agreed at the time to accept a lower amount of child support. I'm not certain about the agreement, but it was probably a full adjudication of the issues of the case, in that you're saying that you're accepting this amount even though you could potentially go for more (obviously you could have).
Now given that his income has increased, you can certainly ask for an increase in the child support amount. Generally, in an action to modify child support, a parent must establish that there has been a substantial change in the income and financial status of either parent, or in the financial needs of the child, since the date of the original support order. If this threshold requirement is met, your ex's obligation to pay child support will be reconsidered under the appropriate child support guidelines. And while you could ask for a "retroactive" support order that would increase it back to the divorce, the court will almost certainly only order it retroactive to the date of filing of the motion to modify child support.
To overcome the settlement where you accepted less than you could have received, you'd have to show some sort of fraud in him reporting his income and that you did not know of his hiding this income (the amount of the hidden income OR the fact that he was hiding it in the first place). IF you can show that, you could potentially get the modification relating back all the way to divorce, but otherwise if you knew that he was at least hiding income but accepted the lower child support regardless, the court almost certainly wouldn't grant the higher support order relating back to the divorce, but only to the date of the filing of the motion to modify.
I know this is probably not what you wanted to hear, but it is the law. I hope that clears things up anyway. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (3 or more stars). Look for the stars on your screen (★★★★★). Thank you, ***** ***** luck to you!