If I didn't claim my son on my taxes last year and his mom received the stimulus package for him, what I need to do when asked how much I received in stimulus?

[Bsch4477]

The stimulus is reconciled on the 2020 return. So if you have not received a stimulus payment for him report that. It doesn’t matter that mom got it for him last year. 

[MinhT1]

You are eligible to receive the stimulus payments for your son if you claim him in tax year 2020.

The IRS says in this document:

Parents who are not married to each other and do not file a joint return cannot both claim their qualifying child as a dependent. The parent who claimed their child on their 2019 return may have received an additional Economic Impact Payment for their qualifying child. When the parent who did not receive an additional payment files their 2020 tax return next year, they may be able to claim up to an additional $500 per-child amount on that return if they qualify to claim the child as their qualifying child for 2020.

In TurboTax, you should say that you received 0 stimulus for your child.

[rjs]

You enter the amount that you actually received (if any). If you are claiming your son as a dependent for 2020 you are entitled to a stimulus payment for him. That's the way the stimulus payments work. The IRS knows that parents who are divorced or were never married can both end up getting the stimulus money for the same child. It's okay.