Your first decision is to find a competent, local tax professional to help you put together his business records.
While he has probably lost the detailed receipts, it is likely that he paid for much of these costs via credit card, so the tax pro can use the credit card bills as a basis for establishing your husband;s business records.
If your husband consistently made money running his charter business, expect the tax bill to be in the tens of thousands of dollars.
Plus, you can expect the tax pro to charge a fee approaching a thousand dollars himself.