As far as residency, The whole year (12 months), Per IRS: to go to school (college) from your residence ( your home ) is not considered as absence.
As far as income: Your son is considered an California resident even though he goes to school out of state. When filling out the personal information section, your son will indicate that he made money in another state.
Enter the W-2 forms for the jobs as they were issued in the Wages and Income section.
If the state that you worked has a personal income tax, most college students do need to complete a state tax return for that state he goes to school in. His earnings in school state should not affect his California state tax return.