My son is really upset. He needs to get better at standing up for himself and asking for help, but I think this is only making it harder for him to do that because he is so discouraged as well.
Keep the onus on the school to resolve this. Do not focus on your son feeling's such as upset or discouraged. Doing so will invite the school to address that as the problem. Focus on the inappropriate behaviors of the bully, and then say no more. Did you write the complete description of the incident, as it occurred, in chronological order? How many numbered points are in the list you created?
When speaking with the school do not make excuses for the bully. Keep your son's feelings out of it. Keep your feelings to yourself. The less said the better.
Separately, at home, address your son's weaknesses. IMO, if there is taekwondo offered in your area and they have a specific curriculum that deals with bullying issues and you already have one kiddo there, it is well past time to speak to them about your son and sign him up. Tell him this will help him learn the skills he needs. Waffling or indecisiveness on your part may role model waffling/indecisiveness to your son, making him seem weak, and causing him to be a bully magnet. This is your private family matter, it does not need to be reported to the school. In other words, your family addressing your son's needs through taekwondo DOES NOT resolve the bully problem or let the school off the hook.