Very good questions from the above poster.
And responding to this boy you know that may be tested for autism or genius--I talk about this in another thread, but children (and adults) on the spectrum of autism disorders have no greater (or lesser) chance of being "genius" or displaying savantism than any one else. The trouble is that its more difficult to 1) gauge intelligence and 2) teach a child on the spectrum, because the communication channels are a different.
Also, with significant cases of autism, some things can appear like savantism out of context, but aren't. For example, I have a student, a boy of ten, who when he came to our school two yeas ago was non-communicating. Now, he's able to communicate to some degree, but it can be difficult to understand why he says things at a certain time and answers to direct questions sometimes don't make clear sense. But, as he learned to speak aloud, it became clear that he had dozens of popular songs of the 60s,70s, and 80s, memorized and eventually started to sing them from start to finish when the mood would strike him. His parents were astounded, having had no idea that the music they played around the house was sinking in to their kid so thoroughly, when things like learning to brush his own teeth had been such a struggle. They bought him a child-size guitar. Here's the harsh truth though: this kid has no musical prowess beyond a normal ten year old kid with an interest in music. His renditions of Thriller and Hey Jude, while adorable and surprising, are not signs of some inner genius, but of a fairly normal level of intelligence we weren't able to see before.
He'd been learning the lyrics to those songs through simple old fashioned repetition, it was just that when he finally learned to express himself, that was where most of his language came from. He has a guitar teacher now and none of us are going to say you're kid's not a musical savant, he's just been listening more than you thought. We're not sure yet whether he understands that the guitar can help create the music that goes along with what he's singing. He's learning scales through repetition and correction, the same as any obedient ten year old can--it's just very difficult to tell whether he knows why. Because while autism spectrum is not, in itself, a learning disability, it's a communication disability.
But from what you describe--if there's some kind of trauma in this character's past, that's different--more along the lines of OCD and PTSD as previous poster said.There's a great book called The Book of Lost Things by John Connolly that deals with a traumatized child (and it's a portal fantasy).