My DS is 18 months old. I brush his teeth every day and night. He has developed a white residue on his tongue, and his breath has a slight odour to it, I can only notice the smell if he is close to my face. He gets cows milk in the morning, and at bedtime, he eats solid foods throughout the day. Does anyone know what this could possibly be? I've tried to brush his tongue gently to see if it would come off, but it hasn't. It's not thick or anything. It's more of a coating