Hello, I have gone through your query and understand your concern. There are plenty of causes for diarrhea. First thing is this diarrhea may be caused by the antibiotic he is taking now. Because amoxcillin may cause diarrhea. Further it may also be caused by infection, food poisoning, Acute gastroenteritis, food intolerance etc. but whatever the cause may be, it should be replaced by enough fluids either by taking plenty of water or Intravenous fluids. It is better to consume oral Rehydration salts available over the counter mixed with water to compensate the electrolyte loss. He should consult his primary care physician, if he have any fever, abdominal pain, bloody diarrhea or black colored stools. This may be a sign of any infection. Hope this helps. Please write back for further queries. Wishing you good health.