That’s a fairly unique situation, I don’t think you will find anyone else who had that kind of experience.

What’s on the birth certificate? Maybe it’s the certificate of citizenship that needs to be corrected?

The concern about credit history is valid, he’ll need to contact all three credit bureaus and ask them to correct his date of birth, to avoid the risk of getting a “fresh start” i.e. a blank credit file with his name and new date of birth on it.