Ok ... that is not how it works ... once a SS# has been thru the efile system once it cannot be allowed again EVEN if you amend the return to remove the child. If I were you I would amend the return again to put the dependent back on the 2019 return and then he will get to file for the stimulus on the 2020 return ... mailing in a stimulus return now is a waste of effort as it will not be processed before the end of the year.
I wish you would have asked these questions before you amended anything.
