To start off with, you don't get that much extra time, it's all a percentage thing. 25% to be exact, for example, if my exam in an hour and a half, i would get around an extra 22 mins.
Secondly, you don't sound like you really know him that personally.
Asperger's is considered a "high function" form of Autism, this means in the basic of terms that we (yes i also have Asperger's) will have a better chance at living independent lives with out constant need of support/care. This does not mean however the person is any less affected by the disability
One of the problems many people with Asperger's face is processing and drawing up information. To the casual observer I would seem to be "fine" and perfectly "normal" when it comes to processing information, but when put in a stressful situation as an exam where I have to write an answer up instead of verbally saying it, it can cause my processing and recall to slow right down. Again this can be a common problem for people with Asperger's. (There's also the possibility that he has something like dyslexia or dysgraphia, which allows him extra time)