California's rule is that any individual who is a resident of California continues to be a resident when absent from the state for a temporary or transitory purpose.
California will consider him a resident until he both abandons his domicile in California and establishes a new domicile in another state. See Change of Domicile on page 10 of this California tax publication for more details:
https://www.ftb.ca.gov/forms/2023/2023-1031-publication.pdf
Also see Leaving California on page 6 of the same reference.
California taxes its residents on all their income, regardless of where earned.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
