How do I prove my ex's income when he owns a business?

If you have not filed a demand for disclosure under Florida Rule of Family Law 12.285 you should do so, also you should serve standard family law request for production upon the other party. These are discovery tools available to you under Florida law which require the production of documents such as tax returns, check registers, bank statements, etc.   Once you have adequate discovery (and you may already have since you have bank records) you should take the other parties deposition, and ask pertinent questions under oath before a court reporter.  You will have to schedule the deposition and file a "notice of taking deposition". You can simply ask the other party questions about the payment of personal expenses from the business both at the deposition, and in front of the judge at the hearing on the case. The Judge will understand that paying personal expenses from a business constitutes income and a smart judge will include that in the other parties income for child support purposes. In the alternative you can have a CPA review the financials and come to court to testify as to what his real income is. You should hire a CPA that has testified as an expert in prior court hearings.