He need to contact the plan administrator at whichever job he wants to have the excess returned to him, and they will compute the amount of earnings. You enter that in the 2020 tax return because it is "for that year" and not "in 2021", even though he gets a 1099-R for 2021. The code on that 1099-R will show it is from the prior year.

I'm pretty sure you have until the tax return filing due date, but I can't recall if that includes Extension.

I like these articles:

https://www.investopedia.com/ask/answers/158.asp