If you are already leaving credits unused, it may not matter. Also, I thought your son was looking at full tuition scholarships, so you may not have $4000 (to claim the refundable AOTC) of QEE.
If you can’t use the child credit in 2019, you might as well ask your ex husband to switch 2019 for 2020, but it may not matter. Can your ex benefit from claiming him in 2019 or 2020? He may not care either.
This will be the first year in 24 years when I have no dependents and no HOH designation. Scary.