My son moved to california from virginia mid year and got an apartment there months later is he a resident or a non resident of california

It depends on why your son is in California.  If it is for school, an internship, or military; or if he is in California only temporarily (generally for 1 year or less on a temporary job assignment); he is considered a nonresident of California.  If he just moved for a new job (or similar), then he is a part-year resident of Virginia and part-year resident of California.

Please note that if he is a California non-resident, then you want to be aware that California and Virginia are reverse-credit states.  This means that all of your son's income would be directly taxable to Virginia (the resident state).  While California also taxes the income, California will give a credit for what your son pays in tax to Virginia on this income.  This is a "reverse credit" because normally it is the nonresident state (California) that taxes first, and then the resident state (Virginia) allows a credit for the taxes paid to the nonresident state.

If he is a part-year resident, he simply pays tax on his Virginia income to Virginia, and his California income to California.