Thank you for your query.
By the description provided by you it appears that your son has injured his gum and which must have given rise to a wound/bruise
which is hurting him. Till you are able to see a doctor kindly apply ice over the area frequently and do not give him warm/hot things to eat or drink. The ice shall stop further bleeding and help reduce the swelling. However examination by a doctor will be the ideal thing.
I hope I have answered your query to your satisfaction.
Wishing your son an early recovery..