He must file a DC tax return. DC regards a person as a resident of DC - and therefore subject to DC taxation - if they "maintained a place of abode in DC for a total of 183 days or more even if their permanent home was outside of DC."
https://otr.cfo.dc.gov/page/individual-income-tax-filing-faqs
He may also have to file a CA tax return. CA regards a person as a CA resident - and therefore subject to CA taxation - if that person is domiciled in CA, but outside CA "for a temporary or transitory purpose." CA's guidelines for determining residency for tax purposes are a bit complicated, and are detailed in this publication: https://www.ftb.ca.gov/forms/2019/2019-1031-publication.pdf
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
