My child has whitish stains on his teeth. Does anyone else's child also have these? Do you know what they could be?

It's a milk stain. Try to brush his teeth after each milk especially at night and this will help to prevent further stain. However the existing stain will not be washed off. Good news is that baby will change teeth when they reach 5-6years old. It is good to start a good brushing teeth habit since young.