You and your son have two different TurboTax Online accounts, don't you? It should not matter that you live in California to your son's return.
This suggest that something on your son's return is connecting him to California. For example, if he filed a tax return years ago with California information, some information carries over, and you can delete it if it is no longer pertinent.
Also, if you have a W-2 with withholding for California - or even just if the state code for CA is present without anything else - this may trigger a California return.
Get your son to open his return (I assume it is Online). Click on State Taxes at the top. Get to the next screen by hitting Continue, then use the delete button to kill the state.
If you are using the CD/download software, then when you are somewhere on the federal return, click on File on the upper left, then in the middle of the list, click on Remove State Return.
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