He will have to file a 2020 tax return, even though he may not be required to do so. If he has no official income to file, he may file a return claiming $1 in interest (which is how the non-filers' returns were reported before the website closed), and this will allow him to receive a credit for the stimulus payment.
For more information, please see this webpage: Non-Filers: Enter Payment Info Here Tool Is Closed