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Yes. If he's carrying accepted photo ID (drivers license, passport, military ID etc.) that confirms he was born 21 years or more prior to that date, he may legally start drinking (and/or buy alcohol) at the stroke of midnight PST.
Whether he will be allowed into an alcohol-serving venue prior to midnight will depend on the nature of the venue (bar, nightclub, restaurant, showroom etc.) and the discretion/integrity of the staff.
Wherever you end up celebrating, we wish him a happy 21st. Have fun but please party responsibly. If you're all drinking, make sure to take public transport, a taxi, a limo (heck, it's his 21st!), or call designated drivers on 702/456-RIDE (7433).