If your baby’s tongue has an awful smell or has a white patch on it then use 100% pure Glycerine with a face towel or cotton wool, if it's for the gum especially when he is teething you can use cotton wool soaked in warm water and a pinch of salt this is to prevent infection on the soars due to the new teeth. But if it's the mouth you are worried about please let the mouth be he is still a baby and on exclusive breastfeeding.