Hello Walamycin is colistin sulphate and advised for children having gastroenteritis with susceptible organisms . Since in your son case the consistency of stool has changed ,so this is working. The normal dose: 5-15 mg /kg/day three times in day till complete cure. Since there are no major side effects ,so you can give it for another 3-4 days after complete relief. Lukewarm water , bananas ( if children is able to eat) , rice curd also help. Still ,no relief then consult your doctor and insists for change of drug. Good luck.