If you mean that your ex-husband will be claiming your son this year, you would indicate that in Turbotax by adding your son to the program as your dependent and answering all of the interview questions correctly. Then when you get to the question below, answer Yes.
Are you letting your daughter's other parent claim your daughter for 2016?
Let us know if you're releasing your right to claim your daughter as a dependent on your 2016 tax return to your daughter's other parent.
Yes, I'm letting my daughter's other parent claim her for 2016.
No, I'm not letting my daughter's other parent claim her for 2016.