Why should he not be an actor just because he is academically gifted? Who says he has to follow a mathematical career just because he's good at the subject? Blimey, I was good at French but I would never have made a career out of it.
If you're really set on the boy taking and using Maths, however, perhaps he could look at something in arts admin and management. That way, he'd get the theatrical satisfaction he craves plus possible breaks if he makes the right moves.
But if he really does want to be an actor then you won't stop him, and neither should you try to. It's not a career to be ashamed of - some of the nicest people I know are actors.