I know someone who was kind of like this guy IRL though not as obsessed. A friend of mine went out to prom with the dude and she clearly told him that there was nothing between them, they weren't going out, etc etc before the went to the dance but the guy still called her up, visited her house, and all that stalker bit for several months after the dance (I don't know if he found someone else, he got the in-your-face-she-doesn't-like-you-in-his-head-finally, or something else happened) before he stopped. And no, the guy wasn't autistic though I know he was bullied when he was growing up.

I think there's clearly something going on with the guy in the story. Maybe he is autistic but then, it could also be that he just doesn't want to face reality because it would mean that he's been rejected or possibly he's thinking that she's just acting shy, or playing hard to get. You will never know why unless you talk down the the person and psycho-analyze him... but even then, I don't think you'll really ever know why. :\