My son is 9 and he lives with me full time. I pay for all of his expenses. Why isn’t he considered my dependent?

There are a couple questions you could have answered wrong under My Info.  Click on My Info then click Edit by Dependents, then click Edit by their name and go though all the screens again.

First of all check their date of birth.  Also you need to enter a ssn for them.

Check the question that says did your CHILD pay more than half of their support.  It's not asking if you did.

If you can't get it to change then try completely deleting them and re-entering them.  Sometimes that helps to clear something out.

And check and make sure you didn't accidentally check the box that says you (or your spouse) can be claimed as a dependent on someone else's return.  You can't claim any dependents if you (or your spouse) is a dependent.   People are saying that if you transferred from last year then one of the spouses came over with it checked that you can be claimed.  Go to My Info and click Edit by your name or spouse's name and uncheck the 3th box on the second screen, it says Someone else can claim you.