I don't know if there is actually anything that stops him (hopefully someone else does), but I wonder how difficult it would be for him to retain specific knowledge for the written exam. Dyslexia is not the transpositioning of numbers, as most people think, but rather a disease which makes understanding and reading written words difficult.
I don't know the boy very well. The few times I've met him, he seemed to be a perfectly normal kid with a flair for drafting and mechanical drawing. He's a freshman at a very prestigious New York City high school (Brooklyn Tech, which is not an easy school to get in to). Judging by his age (13 and already in high school), he may even have skipped a grade somewhere along the way.
I also don't know what was the basis for the diagnosis. I've found from experience in my own family, however, that some diagnoses made by school shrinks seem to be made for pragmatic reasons: The kid needs a little extra help, and slapping a diagnosis on them gets them that help (or more to the point, it gets the school the money to provide it). I doubt that these well-intentioned people have any idea how the diagnoses they make can affect these kids' futures.
I'm not saying that this is necessarily what happened here; like I said, I don't know him all that well, nor do I have any idea what led up to his being evaluated. It could well be legit. But whatever the case, I was asked by his dad what effect, if any, the diagnosis itself will have on his getting a medical. He would like to know that before shelling out the money, and also to avoid getting Brian's hopes up if he won't be able to fly.
Also, assuming that the diagnosis is correct, what effect would it have on his actually being able to fly safely? A few things that come to mind would be memorizing required knowledge (as you mentioned), reading charts, keeping track of traffic, comprehending verbal ATC instructions, setting correct radio frequencies, and so forth.
Thanks again,
-Rich