He's just a boy. Obi-Wan can no longer help him.
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About this quote

Spoken by Darth Vader in Star Wars: A New Hope (1977) during the escalating hunt for Luke Skywalker, the line announces that Obi-Wan Kenobi can no longer intervene. It appears as part of the scenes that tighten the pursued-versus-pursuer dynamic. The terse delivery and the villain's calm menace make the line stick in viewers' memories. Its economy of words adds a cold, final note to the confrontation.