You have a bit of a mix of things in the question. Here's all that really applies:

You deferred an amount owed for the period beginning on March 27, 2020 and ending December 31, 2020. That would be done with the tax return for 2020, being prepared in 2021.

Half was due by end of 2021. Half was due by end of 2022. Nothing here and not anything more, relates to Income taxes. Now it's just deferral payments.

If there was a refund coming for the 2020 income tax return, there would be no deferral of the SS tax owed, because all of it can be considered "already paid in" by the fact that the income tax projection prepaid by estimates and/or withholding exceeded what was owed. That means it is already paid up to the point of what is owed, at least. That's the point of withholding and estimates.

I don't know how loss carryback relates to deferred SS.

"he has received this notice from the IRS stating that he owes $534 and interest. Is this the money that he would have owed by 12/31/2022?"

Questions 20 and beyond:

https://www.irs.gov/newsroom/deferral-of-employment-tax-deposits-and-payments-through-december-31-20...

Yes, they send out Bills. They told us they would do that. No, there should be no interest. Have you confirmed the payment was made timely?