When the 2020 return was filed, the client had an option to report it all in the current year or spread it over three years.  By picking the three year spread, he made an election to spread the income over three years.  I don't think you can change the election by simply reporting the full amount in year two.  Of course my research on this is zilch, so there might be something hidden to allow it, but evidently you weren't able to find that hidden jewel that says "yes".