Thank you for answering my question. I am so sorry your son had to endure this kind of maltreatment and I am sorry the school has not responded well. I'm glad you are asking for help.
First, I think your son need to see a counselor or psychologist to talk about the abuse and his fear. Its important that he get this kind of support as soon as possible. An added benefit of taking him to a professional is you will have an advocate for him to help advise the school on how to help him and encourage action from the school.
With regard to the school, 5 days is more than plenty of time for them to call you back. It important to put everything in writing. Document what happened to him, who was involved, you you talked to at the school, when they said they would call and how long its take for them to call.
Follow the chain of command, fist call the school to inquire about they follow-up call you are expecting and why you have not received a call. If they do not talk to you and/or call you back its reasonable for you to call the principle's boss and so on. Again, write everything down. Do not stop calling and go to the board for help if you are unsatisfied with the reply. Schools often pay attention when parents call the board but I would still try to follow the chain of command first. Also, I think contacting an attorney is probably a good idea. He or she can let you know your legal options. The school has an obligation to protect your son and make him feel safe, that includes protecting his mental health.
Research tells us that our response to bullying must be prompt and firm given the psychological implications of maltreatment. Don't give up.
Here are a few resources that may be helpful:
Good luck to you and your family. My best wishes to all of you.