Answer NO to that question.
If you have been apart from your "almost ex" for at least the last six months of 2020 then you can file as Head of Household and claim the child on your own tax return.
Q. Do I need to fill out form 8332?
A. No. Form 8332 is used when you want to allow the non-custodial parent to claim the child.
Q. Do I need to have an agreement for who would claim him, when he is only my child?
A. You need no form when you (the custodial parent - the parent the child lives with) are claiming the child.
Q. How to answer that question on turbo tax.
A. Answer no. You have no agreement.
Incidentally, for tax purposes, a step parent is the same as a biological parent and the step parent relationship continues after the divorce or separation. So, it's not the fact that you are the only biological parent that allows you to claim the child. It's the fact that the child lives with you.
So, you could allow your ex to claim your child as a dependent. For this reason, you should be aware of the special rule in the case of divorced & separated (including never married) parents. When the non-custodial parent is claiming the child as a dependent/child tax credit; the custodial parent is still allowed to claim the same child for Earned Income Credit, Head of Household filing status, and day care credit. This "splitting of the child" is not available to parents who lived together at any time during the last 6 months of the year; then only one of you can claim the child for any tax reasons. The tax benefits may not be split in any other manner.
Note in particular that the non-custodial parent can never claim the Earned Income Credit, Head of Household filing status or the day care credit, based on that child, even when the custodial parent has released the dependency (on form 8332) to him.