worried about infection

My son has major problems with is teeth. The dentists say that the enamal never formed all the way. He has one tooth with a bad cavity and he complains of it hurting a lot. It has also bleed from the gums right above it. He also grinds his teeth bad at night and there is almost nothing left of the molars in back. He cries a ton and takes tylonel everyday for the pain. He is scheduled to have work done in a few days but I am so worried it is infected. He has been on two bouts of abx already but doesnt seem to help the pain. Would I know if it was serious? Would he have a fever? Can I wait another few days? Can cavitys get infected? Could he be treated at the ER? Lots of questions I know..just scared for him.