The answer to your question will depend upon whether or not your son is listed as a dependent on your tax return, or the tax return of anyone else -- or is instead not a dependent, and thus still retains his own personal exemption.
If the former is true, then the answer is yes, he will have to file tax returns in both California and Oregon, given the dollar amount ($3,480) you indicate as his earned income. If the latter is true, then the answer is no, he will not have to file a tax return in either California or Oregon.
You (or he) can verify this conclusion for yourself by visiting the following California Franchise Tax Board website:
https://www.ftb.ca.gov/individuals/fileRtn/
as well as the following Oregon Department of Revenue website:
https://www.oregon.gov/DOR/programs/individuals/Pages/file-requirements.aspx
Please read those pages in conjunction with the accompanying income charts.
If your son does need to file these state tax returns (because his income is above the filing threshold), then you should also know the following information.
As an Oregon resident, he is otherwise taxed on all of his income, worldwide, no
matter what the source, by the state of Oregon. It is also a fact that income earned in California is considered "California-source"
income; and so that requires paying taxes to California.
Normally, where a taxpayer has to pay taxes to two states on the same
item of income (which often happens when a taxpayer lives in one state
and has a wage-earning job in another state, for example), they are then allowed to
take a tax credit on their home state return for taxes paid to the other
state. Similarly, the same principles typically apply to non-wage
taxation by more than one state.
However, Oregon and California
have a "special" relationship agreement
when it comes to interstate taxation. Instead of the usual method of
claiming a state tax credit on the taxpayer's home state return, instead
the tax credit here is claimed on the
nonresident California return.
While TurboTax can certainly
compute this tax situation correctly, it
can be tricky. The "ordering" sequence of state tax preparation is
crucial. In order to apply the state tax credit correctly, for the
double-taxed income, he would want to complete his federal return
first, his Oregon resident return second, and then his California
nonresident return third.
(Normally, the nonresident state tax return is completed first, before the resident state tax return . . . but not in the case of the two-state pairing of California and Oregon.)
Thank you for your attention, and for asking this important question.
