See "Do I Have to File?" in this CA tax reference:
https://www.ftb.ca.gov/forms/2022/2022-540nr-booklet.html
Even if your son is not required to file in CA, if CA taxes were withheld from his pay then he'd have to file a return in order to get a refund.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
