Yes, your son will have to file a California Non-Resident tax return, in addition to his Illinois state tax return. As an Illinois resident, 100% of his income is taxable by Illinois. However, they will give your son a credit for state tax paid to California. The credit is not a dollar for dollar credit; rather, it is calculated as the amount of Illinois tax that would be paid on the California income. It may not be as much as the California tax, but it won't be more.
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