A possible option to to file the parents' return without the son, then e-file an amended/superseded return with the son.

The new release from last summer seems to indicate that is possible now.

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Taxpayers can also amend their return electronically if there is change to their filing status or to add a dependent who was previously claimed on another return.

https://www.irs.gov/newsroom/irs-taxpayers-now-have-more-options-to-correct-amend-returns-electronic...