My brother since he was a child has diarrhea, and now he is 20 years old and the diarrhea didn't stop. What can it be?

Diarrhea. Your brother may have a food intolerance in the form of lactose, fructose or sucrose intolerance. He needs a workup from a gastroenterologist. .
Malabsorption. He may have a life-long parasitic infection or possibly a malabsorption condition . An example of the latter would be lactose intolerance where the inability to absorb the sugar lactose , present in all diary products will produce chronic , life-long diarrhea. His stool could easily be tested for both of these.