It sound like you need to hire a good attorney who is able to work with a forensic accountant. There are many excellent attorneys who will be happy to offer phone consulations for free.
Like the above poster stated, if your ex-husband is being that difficult with hiding information, a forensic accountant may ultimately be needed. If I were you, I would definitely try to obtain a free consultation and review of your financial statement to at least see if it reveals any discrepancies on its face. Even more importantly, the issue of abuse towards your child needs to be addressed. I offer reduced-fee representation to those with limited incomes, and would be glad to sit down with you to see what can be done for your situation.
I would suggest meeting with a number of attorneys and seeing who you feel comfortable with. Speak to those attorneys at length about your situation and ask them what they suggest in terms of a strategy for handling your case.
Many Law Firms offer free initial consultations (as mine does). I would be happy to speak with you and answer any further questions. Good luck!
Most attorneys will provide an initial consulation without charge. More important, if you have any tangible proof about the hidden monies, it'll help you (and your attorney). Modifications are normally obtained when there is a change of circumstances; moreover, you are entitled to a modification on child support every three years! Good luck.