Not a medical person, but I thought carriers usually didn't get sick with it (or display symptoms of illness). It is possible for family members to be carriers and not know it. I've had friends have this happen. Also - have you gotten him a new toothbrush after every incident? Try doing so if not already. If he gets it again in the near future, ask the doctor about having everyone tested. Or see if he can be tested once he's finished with the antibiotics/symptoms and is healthy.