He may have a file cabinet for financial statements, or a desk drawer. If you have his mail forwarded to you (as next of kin), you should get monthly statements for credit cards and checking accounts and brokerage accounts. If you find a bank account, also ask if there is a safe deposit box. You may get quarterly or annual statements for CD's and IRA and 401K accounts and pensions. You could request a copy of his income tax return; Schedule B would show the sources of Interest and Dividend income.
If it has been a few years since he died, try his state's Unclaimed Property office.