I don't see how the payer could issued a corrected 2016 Form 1099-R indicating that no distribution took since they would have paid the tax withholding to the U.S. Treasury in 2016. Your son could obtain his Wage & Income transcripts for 2016 and 2020, and see what is present with regard to Form 1099-R, but these are unlikely to be updated until much later this year. If he wants to wait and see what these transcripts show before filing, he'll need to request a filing extension.
Assuming that this is still being reported as a 2016 distribution, normally he would file an amended 2016 tax return to claim a refund, but since the statute of limitations is 3 years, the deadline for amending was April 15, 2020 (assuming that he filed his 2016 tax return on or before April 15, 2017), so it's too late now. It's also too late for the IRS to assess taxes on the underreported 2016 income this small. It's entirely possible that the IRS recalculated his 2016 tax liability and found it to be covered by the 20% tax withholding, but if they did, it seems like they might have issued a refund based on that recalculation.
