Your adult son is entitled to a round 3 payment since he was alive for at least part of 2021. The question is how best to obtain it.
While I would generally agree that responding to the IRS letter to re-issue the check would be easier, here it won't work. The IRS can't talk to you about someone else's check, and can't re-issue the check to them if the person is deceased.
In this case, I think you ignore the letter offering to re-send a check, and you file a 2021 tax return with zero income that claims the rebate. You need form 1310 to get the refund in your name, and you need to follow the other steps needed to file for a deceased taxpayer.
https://www.irs.gov/help/ita/how-do-i-file-a-deceased-persons-tax-return
https://www.irs.gov/forms-pubs/about-publication-559
You probably can't e-file in your situation, for at least 2 reasons. First, your son's SSN is probably blocked, and second, even if it is not, you must sign the return as personal representative, and I don't know if you can do that when filing online. You can try of course, but I also believe you need to create at least $1 of income because you usually can't e-file with no income. You can create $1 of bank interest without a 1099-INT, it won't raise any red flags and will allow you to e-file.
