Your son needs to Amend his return to indicate that he is a Dependent in order for you to claim him as a Dependent.  Once his Amendment is processed you ca include him as a Dependent on your return.

In the meantime, you should extend your return due date to October 15, 2019 while his Amendment is being processed.  Click here for instructions:  How Do I File An Extension?

You can click here for instructions on Amending your tax return: How to Change a Tax Return You Already Filed.

Once you begin the Amendment process, follow these steps to indicate your son is a Dependent of Another on his return:

• Login to his TurboTax Account
• Click "Take Me To My Return"
• Click "My Info" from the left-hand side of your screen
• Click "Edit" next to your name
• 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".