First, you might want to make sure you qualify for Head of Household status when you claim your son as a dependent. Just because you claim someone as a dependent doesn’t necessarily qualify you.
Please see the TurboTax Help articles Do I qualify for Head of Household in 2020? and What is a "qualifying person" for Head of Household? for more information.
As far as filing a new Form W-4 each year, that’s probably a good idea, if you can remember to do it. The change would reflect whether or not you had your son as a dependent for the year.
