Error in W-2 reporting to CA & employer not willing to issue amended W-2

Latter of 2020, my son moved to WA for his 1st job & was temporarily using his CA address until he got settled in.  His employer issued W-2 reporting earnings to CA & withheld state income taxes.  Earnings were all performed in state of WA.  Employer is not willing to issue amended W-2.  How do we file his CA tax return & get a refund of withheld taxes but W-2 currently shows otherwise.  Need you assistance please.

Many thanks,

Spencer