If it is your son's intent to return to California then California considers him to be a resident even during the time he is out of the state.
The following is from the California Franchise Tax Board (FTB) and provides detailed information on how California treats residents that are living outside the state
"Section E
Who Are Residents and Nonresidents A resident is any individual who meets any of the following: • Present in California for other than a temporary or transitory purpose. • Domiciled in California, but outside California for a temporary or transitory purpose. See Section L, Meaning of Domicile. A nonresident is any individual who is not a resident
Section L
Meaning of Domicile The term “domicile” has a special legal definition that is not the same as residence. While many states consider domicile and residence to be the same, California makes a distinction and views them as two separate concepts, even though they may often overlap. For instance, you may be domiciled in California but not be a California resident or you may be domiciled in another state but be a California resident for income tax purposes. Domicile is defined for tax purposes as the place where you voluntarily establish yourself and family, not merely for a special or limited purpose, but with a present intention of making it your true, fixed, permanent home and principal establishment. It is the place where, whenever you are absent, you intend to return. The maintenance of a marital abode in California is a significant factor in establishing domicile in California."
California uses domicile as the primary factor in determining residency statue. This is a complex matter and I have provided a link to the FTB web site addressing the specific questions you have.
Link to FTB site Guidelines for Determining Resident Status
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