I paid a deposit of $500 in November of 2020 for my son to attend a college beginning in the fall of 2021. Is this deductible for either the 2020 or 2021 tax years?

[DawnC]

It should be part of your son's 2021 Form 1098-T that will be reported on your 2021 tax return along with the rest of his education expenses assuming you will be claiming him as a dependent.   Since he will be starting school in the fall of this year, 2021 will be the first year of his educational credits.  

Guide to Tax Form 1098-T: Tuition Statement

The Lowdown on Education Tax Breaks

[Hal_Al]

Technically, no. Payments must be for the current year or the first term of the following year (starting Before April 1).

But, as DawnC said, if they include it in box 1 of your 1098-T, you can probably count it.