How does my son complete the 1040X to amend that he can be claimed as a dependent? He did not check the box on his original return. 1040X is confusing for this situation.

By the way, did your return reject because he claimed himself?  Just checking, you know you will have to mail your return this year.

To indicate that your son can be claimed as a dependent when amending his return. visit the My Info section by clicking on My Info in the left-hand column. On the Personal Info summary page, click Edit next to his name and change the check boxes in question 3. A few other things we need to know as in the attached screenshot.

This is all you have to do if you do not have any other changes.