Make a case to impute income based on ability to earn and job skills.
The answers on this discussion board are general in nature and NOT intended as legal advice. Responding to questions does not constitute an attorney-client relationship. Always see a lawyer about your individual situation.
Contact an attorney in your area. Many, such as myself, offer free consultations. You can file for modification and then use the discovery process to obtain tax returns. You can also ask the Judge to impute what he is capable of earning. Many times parties try to use the excuse that they do not have a job. Often times the courts will look at what the parent does for a living and then award support based on the amount they could be making.
There are several things that can be done. Contact an attorney.
Note this answer does not constitute legal advice, and should not be relied on. Each situation is fact specific and court specific, and it is not possible to evaluate a legal problem without a comprehensive consultation and review of all the facts and court pleadings filed in the case. This answer does not create an attorney-client relationship