I agree with the comments about the kid seeming too grown up, but not in the same way. His actions seemed pretty in line with the plausible; I work with kids and know that in certain extreme situations they can be frighteningly together. No, what bugged me more was from the very start the dialogue and inner monologue. While I am fully on board with "kids are more tuned in than we think," there is the plain fact that 9-year-olds, even realy mature ones, don't talk like adults. I know that this is just an extension of the phenomenon of fictional characters not talking like people really talk (perfect complete sentences unless they're being characterized as stupid or rural, lack of "umms" and repetition, etc), but for some reason I found myself less able to get past the unrealism.
Still, great story. I really like the "fresh perspective on overdone horror scenario" angle.