Even under good circumstances many people, especially men, do not like to go to the doctor or dentist. They really just do not handle that type of pain as well as women.
Have you spoken to the dentist about what type of pain medicine your son will be given? Or what he may suggest as an alternative to your son being just given the local shot. I know when you have your wisdom teeth removed they can put the patient under anesthesia.
Also maybe have your son visit the dentist and have a conversation about what exactly he can expect. And what will happen if he does not have the cavities filled.
Have you asked about maybe giving him an anti-anxiety medication for the day of the procedure? Maybe also speak with your sons regular doctor about what he suggests? Just make sure you also work with the Dentist.
There are natural things your son could take to help him not be so stressed. They have natural herbs, but again you will need to let the dentist and physician know about them since they can have an effect with certain medications.
You are the parent. You must stay calm, you also are not alone. Many children do not like to not "be in control" even without his past experience. The more you play into his fears, the worse it will be. YOU need to be self assured and in control. Be honest with your son. My mother used to always say, "there will be some discomfort, but it will not last forever." "She would say "the more relaxed you are, I promise the less pain you will have." She was right. The first time I just took long breaths, and calmed down was the first time I realized shots did not have to hurt! I think being tense was what was causing the shots to hurt.