My first suggestion is to get the dog to a vet as soon as you possibly can for a full examination - it may be in pain and fearful of your son hurting it more. I really hope that this is the case - a weird thing to say, I know, but you are playing with fire otherwise.
Keep the dog muzzled or in another room to prevent any accidents that could have very tragic results until the it has been assesed by a vet.