If he is under 19 on December 31, then his student status is not an issue. The factors (besides age and relationship) are,
•residency--does he live in your home more than half the year not counting temporary absences
•support--does he provide more than half his own support.
You would need to add up his support costs, including food, clothing, transportation, entertainment. Include the fair market value of rent for the home you provide and an equal share of utilities. If he provides more than half his own support, he is not a dependent even though he meets the other tests.
