If you know him, you probably understand why he will blame anyone but himself for not knowing the nature of this inherited investment, then failing to mention them to you.  Ignorance is often difficult to acknowledge.  But why would you want to put 2021 and 2022 income on a 2023 return?  If it were any other kind of income, would you do that? And let's not blame IRS for not informing him until the bonds were cashed that there is taxable income to report in the year they matured.  The bonds weren't in his name, and they probably didn't have the father's SSN. 

You can tell him he should file amended returns.  Depending on the amount involved, you can mention that he might expect a CP-2000.