hello and welcome to HCM forum,
i am glad you wrote in, i would like to inform you that sometimes, caries or even trauma to the tooth can cause infection, which results in an abscess
, usually it is painful but in your son's case its not, therefore in order to confirm, you will have to visit a dentist
and get it checked clinically.
you have not mentioned anything about the color of his tooth, has it turned dark, if it has then it suggests an abscess. but you need not worry, your dentist will take care of it.
i wish your son good health,