I have just started treatment for an ASD called Aspergers Syndrome. Even though it makes science and maths quite easy. It makes subjective subjects like english really difficult. Your frustrations are based on the fact that this person is really intelligent and gets extra time which you believe to be unfair however, something you don't understand is that for maths and science he will probably get top marks however for english he will (probably) struggle as for people like us, it's difficult to understand how people operate and so trying to understand the author is an almost impossible task. Then you have to consider that you only get an hour and half to figure this out. Extra time is necessary in this circumstance. However, you can't just give him extra time in 'some' exams, you have to give him extra time in all exams because that's not fair on him to only have extra time in some exams.
Finally, I would like to point out that exams aren't the end point of your life. You do your GCSE exams and then you do your A-Levels. Then, you may go to university. However, as soon as you get out of education, have you considered how difficult it will be for that person after he leaves school and he has to live in a world he doesn't understand?
Just something to think about.