(1) Yes, he is a California resident. If he lives there when not in school or summer undergrad work, has a Driver's License, Voter's Registration, Bank Account, etc. there. You normally don't become a resident of another state unless you intend to establish residency there. New York is an exception; if you live there over 184 days, they consider you a resident.
(2) He should file a Non-Resident return to both New York and New Jersey, since both states deducted taxes on his income. Prepare the Non-Resident Returns first. The address on the W-2 is irrelevant to the IRS; only the Mailing Address you enter on your return counts.
(3) Basically, ALL his income is taxable in California; his resident state. In the California interview, he will be asked for 'taxes paid to other states' and receive credit for the taxes he paid in NY and NJ, against the tax CA would assess on his total income.
Here's detailed info on How to File a Non-Resident State Return and Multiple States to help you.
@flyerii
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