Child Support: Ex withholding wage information?

I have reason to believe that my ex is withholding his commissioned wages. I believe he only reported his hourly wages. I called my case worker and learned what my child support payment would be and I know he has not reported his income correctly. I know how much he makes hourly+commissions and my child support payment of $398 per month cannot be correct. He makes over $4,000 a month. Is it possible that he has only reported his hourly wages to the state??? Can I subpoena his W2's or tax returns?