Only one needs to be correct ... so did he file a 2020 return ?
If so look at line 8B of the form 1040 for the AGI ... do not use the amount on an amended return.
If he only filed a non filer's return then use $1 for the AGI.
If neither of those work then use a single zero for the AGI.
If all of these fail then he will need to mail in the 2020 return.
