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We checked with both Metro and some random hotel security officers and, as suspected, they all go by the date of your birth, not the hour, so your son will have to wait until midnight in Las Vegas to enjoy his first legal adult beverage, or to hit the tables and machines.
If you're anxious for some adult fun prior to that, you could always check out any of our fair city's all-nude strip clubs (aside from the Palomino, which serves liquor) -- since they can't serve booze, the door policy is 18+. Why not get him a copy of Topless Vegas as a fun and useful gift that will help him navigate Sin City without getting ripped off?