Other mistake: In one of the opening scenes where the boys ride to the baseball game, the fishermen they pass make a rude comment towards them. In response, you hear Joe ring his bicycle bell and see them flip the men off. The bicycle has no bell.
Other mistake: In the second baseball scene, Joe is sitting on the bench next to one character with the number 8 and Simon, who is on Joe's other side is also number 8. Two players on the same team would not be the same number.
Answer: I don't know if it was ever stated what Simon died of, but Simon - against the odds - lived much longer than the doctor thought he would. Simon continued to "outgrow his body" (internal organs became too large for his tiny body). It can be assumed that Simon contracted pneumonia or hypothermia from being in the frigid water when he rescued the children from the submerged bus. I think Simon - expected to die at a very young age - was staying alive until he could realise his purpose or "God's Plan" for him. To "believers", Simon was "made small" and practiced holding his breath underwater (seemingly for no apparent reason) so that he'd be able to save the kids on the bus... then, having fulfilled "his purpose", was called "home."
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