Based on the little information you provided, your client is a domicile resident of CA and is in AK only for temporary and transitory periods.  On that basis, he remains a tax resident of CA and, as Lisa says, is subject to CA tax on worldwide income.

Since there is no personal income tax in AK, he will just need to report his wages and pay tax to CA as usual with no credit for AK tax to offset his CA liability.  Note also that you should not rely on Box 1 of W-2 to report CA wages because CA does not fully conform the IRC; you should review details of your client's W-2 breakdown and adjust for non-conformity items such as HSA accordingly.