At this point, in order for you to claim your Son as a Dependent on your return, his return needs to be Amended first. You can click here for instructions on Amending a tax return: How to Change a Tax Return You Already Filed.
Once he begins the Amendment process, he will need to revisit the "My Info" section and click "Edit" next to his name. As he walks through the TurboTax interview process, he needs to answer "YES" to the question "Someone else can claim me as a Dependent" and "YES" to "and this person will claim me on their 2018 Tax Return".
Once his Amendment has been processed, you will be able to e-file your return. In the meantime, you can click here to extend your return: https://turbotax.intuit.com/irs-tax-extensions/
Alternatively, you can mail in your return.
