Thanks to both of you for the information. So the sequence of events should be that I e-file an amended 1040-X straight away, removing him as a dependent, and he should mail his own return as an independent filer, also today. Will the IRS then, over the next 12 weeks or more, reconcile the new filings with my already-accepted but incorrect return? I suppose I am concerned that the IRS might reject my son's mailed return because my incorrect return had not been updated yet in their system.
