First, we should figure out the facts, so we're not all just talking out of our hats. Then, we should figure out the legalities, which will tell us whether he should be charged with treason or some other offense.

Assuming we conclude that he is not insane, and that he willingly took up arms against the US, we should prosecute him to the fullest extent of the law. He's young, but old enough to know better. I don't see any justification in letting him off just because we've got a soft spot for American kids.