Have him show you where hes reading on the IRS website that agrees with him.

Hes not paying taxes on them twice, he had withholding on the total but all the withholding was used up in year one.

IF you amended 2020 to spread out the withholding (which you can't, that's no how it works), then he'd owe back that withholding that was claimed in 2020, so his plan doesn't help him.