Does he own stock in the new bank?  Did he receive any money when the bank was sold?  Was this a merger, or is Big Bank now just a stockholder (maybe the only one) in Little Bank?  If he was on the Board, that means he was a Director, and any compensation should be reported on a 1099.  Sounds like some records from 18 years ago are missing.  However, he may not remember that along with buying stock, he got an option for more shares. 

This is from the IRS audit manual on such stuff:

 Former employees’ compensation should be reported on Form W-2. A reconciliation should be requested for  some of the larger exercises to the employee’s reported option income in Box 1and Box 12, code V of Form W-2.  Extra steps must be taken to reconcile deductions to the proper year for companies with a fiscal year end.  Discrepancies in the reconciliations may indicate an income or employment tax issue.  

If the options are offered to directors, ascertain whether a Form 1099 was issued. This option income should be reported on the director’s individual tax return (e.g. Schedule C or on line 21, Other Income), along with self-employment tax upon exercise or other disposition.

https://www.irs.gov/businesses/corporations/equity-stock-based-compensation-audit-techniques-guide