Simple answer: If he is your dependent, he is a resident of the same city you live in and is obligated to file a city return, for that city, if he has earned income.. He is NOT a resident of the city where the college is, even if he is living off campus.
That assumes he is your dependent, under the "Qualifying child" rules (as most [almost all] college students do). If he is, instead, a qualifying relative, then he might be considered a resident of the college location city.
