Child support will be based on his tax return. If it is less the last year, if you request, they may average the last 3 years. Finding unreported income is another story. It's very difficult to prove and usually takes a forensic accountant, and from what you described, would cost $10K or more, which you would have to pay and there's no guarantee they would find anything.
This isn't anything new. Under reporting income on a 1099 is probably the most common way folks pay less support. It's just very difficult and expensive to prove. If you haven't already, it woudn't hurt to a consult with a local attorney. Some judges may be more harsh with less evidence and an attorney would be more aware of the prevailing attitude in your local court.