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.
The whitish stains could be from eating toothpaste unintentionally while brushing his/her teeth. To prevent further stains, tell him/her to not eat toothpaste/ change the flavor of the current toothpaste you're using.