Why didn't Brian react to the plane or pilot?

I believe that this question is asking about the sequence of events that occur in chapter one when the pilot of the small plane is having a heart attack. Brian doesn't react much to the pilot for two reasons.
Brian's mind is elsewhere. He is constantly thinking about the divorce that his parents just got. Furthermore, he knows the reason, and it is devastating to him. Brian is essentially still in shock at the entire situation.
Brian is young and not likely to recognize the signs of a heart attack. The pilot is showing classic symptoms of a heart attack. He's got bad gas. Surprisingly, indigestion is a possible sign of a heart attack. If that were the pilot's only symptom, then Brian would be right in thinking that the pilot probably ate something that doesn't agree with him; however, the pilot is showing other signs as well. He keeps rubbing his shoulder, and he's wincing in pain frequently.
If the question is asking about why Brian doesn't react once the pilot becomes unconscious, then we have to look at the start of chapter two. Brian doesn't react because he simply can't believe the situation that he is in. Additionally, Brian tries to convince himself that his situation isn't actually happening:

Then he looked for ways for it not to have happened. Be asleep, his mind screamed at the pilot. Just be asleep and your eyes will open now and your hands will take the controls and your feet will move to the pedals—but it did not happen.