Our 10 year old thinks a classmate's extra time in tests is unfair

Our elder son came home annoyed that a classmate gets extra time and a separate room for tests. In his words, 'he gets to finish and I don't'. He likes this boy, which made it more confusing for him. I tried explaining that some children need a different setup to show what they know, the way someone who can't see well needs glasses. He listened, then pointed out that glasses don't give you twenty extra minutes. How have others explained this to a child his age, without saying anything about the other boy that isn't ours to share?