My son and I are Michigan residents, but he went to school in California for 2018, and I have 2 W-2's from California. Do I need to file state taxes in CA or Michigan?
Yes, you will have to file both.
You have California source income.
You must file a Michigan return if you file a federal return or your income exceeds your Michigan exemption allowance. A return must be filed even if you do not owe Michigan tax.