I am sorry I misread your earlier post. I believe that your son should file an amended return. As I stated before, I do not know whether this will provide him with a stimulus payment this year. It takes 6 weeks for an amended return to be processed.
If you are having trouble with the online version producing a 1040X, I would download the software and force the 1040X to be produced in Forms Mode.
To download the software you can sign in to your sons' account and in 2019 select Amend and then choose the Download software. You can also download it here. You will also need to download a .tax file. Open the .tax file and go through the amending process. If the system is not producing a 1040X, in Forms Mode use the search to find the 1040X.
If you want to file the 1040X by hand, that is your prerogative. If your sons' return is fairly simple, it should not be a problem. You can find the 1040X here and the Instructions here.
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