If he actually moved to California during the tax year, then he would file part-year resident returns in both MN and CA. He has no tax obligation to the state of Illinois.
But if his main, primary home remained in MN all year, then he would file only in MN.
The "Amtrak Law" of 1990 prohibits the compensation of any rail or motor carrier employee who works in more than one State from being subject to State income taxes in any State but the State in which the employee resides.
https://www.congress.gov/bill/101st-congress/house-bill/5075
If his employer withheld taxes for the wrong state(s), then he would have to file a non-resident tax return in those state(s), showing zero income, in order to have the incorrectly withheld taxes refunded to him.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
