My son gets diarrhea from whatever fruits he eats, only exception is apple.

High sugar & fiber: All fruits have fructose, a sugar that can be hard to digest, like lactose in milk. In addition, they both have a high water and fiber content, which tends to give a softer, larger stool. The amount and type of fiber varies with each fruit, so some are more problematic than others. There is no treatment other than adjusting intake.