As a student, or member of the US Military, he wouldn't need to register; he even can pay his taxes as a Non-Resident. However, anything he makes, part-time or otherwise, while in California is taxable income; you need a tax advisor if he has capital gains or dividend income..... Even golfers that live in Florida pay California income tax on their winnings for tournaments that are held in California.
But if he has a full time job, he's a resident, and needs a Cal drivers license and auto registration, insurance (must have proof of insurance with a California address to register a car), etc.
Congratulations and welcome to the sunny state.