White stool indicates lack of bile, that is produced by liver, stored in gallbladder and excreted in small bowels to help the digestion.
If the liver doesn't produce bile or if bile is obstructed from leaving the liver, stool will be white.
This can happen when there is an infection of liver, like hepatitis, biliary cirrhosis, gallstones, anatomic abnormalities of the intestines or bile ducts present at birth, narrowing of bile ducts, tumors and as a side effect of certain medications (antibiotics, antifungal drugs, antacids).

Visit his/her pediatrician!