How do I handle a bully half my age?

I am almost 28 and a friend of mine's nephew is 14 and he is bigger and stronger than me. I am 28 and only 5'6 and this kid is stout.

What can I really do because he is underage. I know he is just trying to mess with me, but he punches me on the arm in the same spot and it is painful. I never fight back because he will just hit harder. He knows how to fight while I don't.

How do I handle a bully half my age?
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