You might need professional help for that.
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My non-legal take is that he remains locked as a CA resident (but filing as nonresident just for his military income) until his final release after the 3 Mos was up. Thus, not really ID resident unless he submitted to military HR a DD2058 changing it to ID before his terminal leave was up. Then Part-yr CA until that date, and Part-YR ID after the end of terminal leave.
Wrangling the rest of who owes whom for civilian income before those 3 mos. were up, might take a local pro to figure out. I'd think it would be ID Non-resident income until the 3mos were up, then ID resident income after....and Might have to claim CA income too for those 3 mos.
BUT, Absent the DD2058 before the 3mos were up, that would mean he would not be an ID resident at all for 2024, and would file ID only as a non-resident for 2024.
But that's my take on it, and could be wrong. Giant mess...get a local "experienced" pro to do it this year. Yep, will cost more.
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Of course if he did file a DD2058 with the military, changing state to ID BEFORE the 3 mos were up, then he became a resident of ID as of the date the DD2058 was "accepted"/signed off by Military Authority.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
