My 4 year old son has diarrhea since sunday. the gp prescribed kantrexil since tuesday but it still hasn't cleared up. what to do next?

Missing info: Diarrhea in kids is most often caused by self healing viral infections. Few are caused by bacteria that require treatment & why the kid was on this powerful antibiotic is not clear. Many improve simply by adding breads/bananas/applesauce and rice to their diet. Given the lack of detailed information not much more can be said.