It is likely that your regular water is not entirely safe for drinking. While adults have more immunity and are able to drink this without any problem, a young child with relatively less developed immunity is falling sick. Make sure that your drinking water is taken through aquaguard/reverse osmosis kind of water purifiers available these days. Also ensure that other foods he takes are fresh, hygienically prepared and served, and that his nails are cut and hands washed before and after he eats.