When you entered him as a dependent in the My Info section, on the screen where you select the months he lived with you, you were notified on the screen that for a child born in 2019 you select the Whole year.

Go back to the My Info section, edit the dependent and correct the mistake.

Do you mean Sept 2019 (not 2018)?

Or if he was really born in 2018 did you accidentally type the date of birth as 2019?