It depends. The key is where is "home". One thing is certain: he is not considered a California resident because he is only present in California for school. He will prepare a California nonresident return for all of his income in California.
However, his other state of residency depends. It sounds like it is Washington based on the description you are giving. But the question is if Washington was his state all year or part of the year. If Washington was his resident state before entering school, and at least one of you is maintaining Washington residency, then he is a Washington resident for the entire year. Only one return would be filed: California nonresident.
However, if he did not have Washington residency entering school, it will be difficult to now claim full-year Washington residency. In this case, he is a part-year resident at most. He really would need to file a Hawaii part-year return or even a full-year resident return. If he is a Hawaii resident, then Hawaii also taxes the income he earned while a Hawaii resident, but will give him a credit for the taxes he pays to California on the same income.
It's not a clear-cut case by any means. Needless to say, if you can justify Washington residency then this is a much more advantageous way to file. Maintain documentation for the position you claim on the return.
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