Thanks for posting your query in healthcaremagic.
If the fractured jaws has not completely fused even after bone plating,it can cause pain.
AN OPG radiograph as well as clinical evaluation like palpation will help to rule out the cause of pain.
Meanwhile,i would suggest you to take augmentin
along with analgesics.
If necessary,another surgery has to be performed.
Avoid mouth opening too widely.
Jaws has to be immobilised for a while.Physiotherapy
may be helpful.
I wish your son speedy recovery.