He will remain a California resident for tax purposes until he establishes his domicile in NV. See page 10, Meaning of Domicile, in this CA tax publication:
https://www.ftb.ca.gov/forms/2021/2021-1031-publication.pdf
As long as he is a resident of CA, all his income remains taxable by CA. After he establishes NV as his state of domicile, he can only be taxed by CA on California-source income, for example income from a job he actually performs in CA or rental income from a property located in CA.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
