In order to obtain a refund of the withheld CA taxes, your son will need to file a non-resident CA tax return, Form 540NR, showing zero CA income.
If he has "abandoned" his home in CA, now makes his permanent home in DC and has no intent to return to CA, then he has made DC his domiciliary state and he is no longer a resident of CA. If CA questions his residency status (which they might), I would tell them that he has literally moved to DC with no intent to return to CA. And it would help his case greatly if he can show them that he has a DC drivers license, DC license plates on his car, and is registered to vote in DC.
And if he hasn't done so already, he should ask his employer to start withholding DC taxes.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
