My son just had root canal four days ago but still have toothache. what could it be?

Call dentist: This is hard to answer on many levels. One is was there a large infection at base of tooth? Was there continuous pain prior to root canal? Is there periodontal infection as well? How do his teeth mesh together? Is there a fracture in the root? Is he taking an antibiotic? Is the filling high and hitting first? Your son should go back to the dentist to get this looked at? Any post op analgesics?

See endodontist: If you experience pain after an old root canal procedure, you may need endodontic re-treatment. Endodontists are dentists with 2-3 additional years of specialty training in root canal treatment and saving teeth with a 97% success rate..

Call dentist: Call the dentist/root canal specialist and inform them of the pain issues. Discomfort is normally present for a short time after RC treatment, but 4 days and a toothache is probably a situation that it should be re-evaluated.