I am sorry to hear about your son. In all probability he had a dental infection below the tooth that has the silver filling. This infection has spread to jaws and gum. Other than this he could also be having a gum infection, an infection of the skin of face, a boil or abscess or infection of the jaw bone.
It is difficult to diagnose this on net. Please consult his pediatrician and his dentist as soon as possible (read immediately!). Meanwhile please give him acetaminophen to make the pain tolerable. Take care!
please do not give your child Anything---please---until you see or talk to your doc who knows your child. there could be a problem with acetaminophen. There have been problems reported. OK?