It depends. It does not hurt to make the estimated tax payments to California if he prefers to.
If his state tax liability for 2025 is expected to be over $500, and the amount of his withholdings is the smaller of 90% of the 2025 tax or 110% of his tax as reflected on his 2024 tax return, then he should make estimated tax payments. The installments paid are typically 30% of your total expected tax for quarters 1, 3, and 4, while 40% would be due with the 2nd installment.
Please see this link from the Franchise Tax Board for more information on estimated tax payments. The payments can be made directly to the Franchise Tax Board using the instructions.
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