certain foods and medications can cause that, ESPECIALLY peptobismol will cause black stools large stools can be from constipation where the fecal matter just keeps piling up and getting bigger until he has a bowel movement to push everything out
Large stools don't indicate a problem; everyone is different in the size of their stools. However, black stool and diarrhea indicate a problem. Black stool usually means that there is blood in the stool. He should go see a doctor to get evaluated.