If you claimed the AOTC for four (4) years on your joint return and he is no longer a dependent, then just delete the Form 8863 from your tax file - it no longer applies.
See: How do I delete forms in TurboTax Online?
After deleting the Form 8863, log out of the program. When you log back in, use the Delete a Form tool (List of Forms in My Return) to confirm it has been removed from your return.
Why does this happen? When you transfer the information from your prior year return, the program automatically includes certain forms in the current year - to make sure you don't forget to update those forms. If a form is no longer required (e.g. K-1s, Schedule C), you have to delete them from the current year.
