@Critter-3 Do you have a reference source for your answer?
I'm of the opinion that, to be technically correct, poster must file an amended return for the year the bonds matured and report the interest for that year. But I can find no reference that says that directly. I base my opinion on the wording in Pub 550 (see below).
As practical matter, reporting the interest in the year the bonds are cashed is probably good enough since he will get a 1099-INT for that year.
From page 7 of Pub 550, emphasis added https://www.irs.gov/pub/irs-pdf/p550.pdf
Reporting options for cash method taxpayers. If you use the cash method of reporting income, you can report the interest on Series EE, Series E, and Series I bonds in either of
the following ways.
Method 1. Postpone reporting the interest
until the earlier of the year you cash or dispose of the bonds or the year in which
they mature.
Note. Series EE bonds issued in 1990
matured in 2020. If you have used method
1, you generally must report the interest
on these bonds on your 2020 return. The
last Series E bonds were issued in 1980
and matured in 2010. If you used method
1, you generally should have reported the
interest on these bonds on your 2010 return.
2. Method 2. Choose to report the increase
in redemption value as interest each year
