Hello Raja! The stool examination showing presence of reducing substance suggest that there is some types of unabsorbed sugars which is not digested and absorbed in the intestine as such it can be lactose present in milk, it can be carbohydrates also. Moreover as starch granules are present that he may be having some carbohydrate malabsorption
. I will advise to visit you a Gastroenterologist who may advise your son few blood tests like Antibodies
levels, Lactose tolerance test, basic blood investigations to see for malabsorption and may be endoscopy
examination and duodenal biopsy
for confirmation of diagnosis.I hope I have answered your query and this will help your son. Remain in touch.