、？
What's the matter, little guy?
Literal
[You-]ぼく, how-did [nominalizer-の]?
A compact exchange with a notable pronoun reversal: the addressed child is being called ぼく, because an adult can affectionately refer to a small boy by adopting the boy's self-reference ('hey little-guy, what's up?'). どうしたの is the standard casual 'what happened?' question, the nominalizing の functioning as a soft question particle.