You can put your son on your tax return even though the other parent is claiming them, so you can file as Head-of-Household (HOH), if this applies to you. Just make sure to indicate that Someone else can claim child on their tax return AND Someone will be claiming child on their 2016 tax return. To do this go to the Personal Info section and select the Edit button to the right of your Dependents. On the next page, select the child that you want to allow the other parent to claim and click Edit next to their name. Go through the interview questions for that child and check the box next to Someone else can claim this child on their tax return AND check the box Someone will be claiming child on their 2016 tax return. This will make the child non-deductible for you allowing the other parent to claim the child as a deduction on their tax return.
