You can only claim HOH if the child physically lived with you more then half the year.

There is no such thing in the Federal tax law as 50/50, split, or joint custody. The IRS only recognizes physical custody (which parent the child lived with the greater part, but over half, of the tax year. That parent is the custodial parent; the other parent is the noncustodial parent.)

Who can claim the exemption and credits depends on who is the custodial parent. (By the IRS definition of custodial parent for tax purposes - this is not the same as the legal custody that a court might grant.).

The test that the IRS uses to determine the custodial parent is where the child lived for more than 1/2 (or greater part) of the year. The IRS will go so far as to require counting the nights spend in each household - that person is the custodial parent for tax purposes (if exactly equal and more than 183 days - The custodial parent is the parent with the highest AGI, if less than 183 days then neither parent has custody so the child cannot be claimed by either parent). And yes they are that picky.

See Custodial parent and noncustodial parent  in Pub 501

https://www.irs.gov/publications/p501#en_US_2018_publink1000220906

Only the Custodial parent can claim: (Child would be listed as non-dependent EIC & CC only)
-Head of Household
-The Earned Income Credit
-The Child and Dependent Care Credit
-The Health Coverage Tax Credit

The non custodial parent can only claim: (Child would be listed as dependent)
- The child as a dependent
- The Child Tax Credit or credit for other dependents

But only if specifically specified in a pre-2009 divorce decree, separation agreement or the custodial spouse releases the exemption with a signed 8332 form - after 2009 the IRS only accepts a signed 8332 form that must be attached to the non-custodial parents tax return.

But assuming that you have the same filing status every year (as macuser pointed out), you still may want to adjust your W-4 each year to include (or remove) the dependent.   If the dependent is under age 17, keeping the dependent on the W-4 during a year that you do not claim the dependent would result in about a $2000 difference in taxes.

@macuser_22 Thank you for your response but I think you misunderstood what I was asking. I understand how the IRS determines who is the custodial parent each year. I claim him in odd years, per our parenting contract, so I list him as living with me 7+ months out of the year. In those odd years (such as 2019), I use the HOH filing status. But when his mother claims him (for 2020), I use the single filing status. That is not the problem. My question has to do with the W4. The new W4 only asks you what your anticipated filing status will be. Where I will be claiming his for the 2021 tax season, I am thinking I should select HOH. My question is do I need to fill out a new W4 at the beginning of 2022 and select the single filing status since I will not be claiming my son for that tax year?

@AmeliesUncle I was wondering how each filing status would affect my tax withholdings each year. To be honest, I can't remember what I chose on my last W4 and I ended up not having any federal tax withheld for 2020 which resulted in me having to pay taxes this year. I am trying to get this straightened out with my employer to see if a mistake was made.

Thank you both for your responses. I am just trying to make sure I am getting the correct amount withheld to avoid paying again. (FYI - the W4 no longer asks you how many exemptions/allowances you would be claiming. Something I didn't know until now.)

The only way your situation as you described is legal, is if you and the other parent actually adjust the child‘s living schedule so that it physically lives in one parent’s home more than half the nights of the year, and the next year it physically lives in the other parent‘s home help more than half the nights of the year.

In that case you would switch from filing head of household with one dependent, to single with no dependents, and that will cause a very large swing in your tax bill and it would be a very good idea for you to file a new W-4 each January.