You say he only made $4K. Is that exact or rounding? The rule for being a qualifying relative dependent is that gross taxable income must be less than $4200.
So, if he had less than $4200, he is your dependent and if lives with you, he is also your qualifying person for Head of Household.
The term "exemption" is no longer used, starting in 2018. He is just your dependent. Instead of the old $4200 exemption deduction, you now get an up to $500 tax credit for an other dependent.
