He would no longer qualify as a child since he is over age 18 and not in college. He may, however be a qualifying relative. See the qualifications below.
You can claim a child, relative, friend, fiancé (etc.) as a dependent on your 2019 taxes as long as they meet all of the following requirements :
- You provided more than half of their financial support. More info
- They made less than $4,200 in gross income during 2019 unless they are a qualifying child.
- They live with you or they are related to you. (Your relative must live at your residence all year or be on the list of “relatives who do not live with you” in Publication 501.)
- They are a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
- They aren't (or won't be) claimed as a dependent by someone else.
- They aren’t filing a joint return with their spouse.
- You are not being claimed as a dependent on someone else's return.
For Head of Household, the IRS uses this test :
"he or she lived with you more than half the year, and he or she is related to you in one of the ways listed under Relatives who don't have to live with you , later, and you can claim an exemption for him or her"
See this link for more information: Pub 501