Suggest Remedy For Stomach Ache With Black Or Red Stools

my son 8 yr. old boy had a stomache and diarrhea his stool is black and soft and many. he also experienced amoebiasis last july.he is complaining that his stomache ache is coming from the upper abdomen at the center but it last for seconds after he goes to toilet. his pedia prescribed metronidazole when he had amoeba. I used it now. it stops his diarrhea but his stool is soft and black sometimes yellow i observed it this last week then this evening he again complain that he had stomache and his stool is already bright red.