I'm sorry to hear that school officials aren't listening to you. Since you haven't given details about what is going on in the school, it is difficult to give you specific suggestions. Here is a list of some things you can do that might help.
Keep a journal. Write down dates, times and a detailed description of events. Stick to the facts and try to keep emotions out of it. You may need to prove what is happening and the more accurate you are the better. Be sure to include that letter that the teachers gave your son.
Get eye witness accounts. There might be other teachers or parents who have seen exactly what happens. Ask them if they will help you put this matter to rest.
Go to the school board and district offices. If they are not listening to you on the school level, then it is time to step it up and let the school board members know what's going on. You can either call their offices or send an email to all members describing what is happening. Ask them what they are going to do about it. Ask them what the school district's policy is on bullying.
Here are some more drastic measures - hopefully you won't have to go this far.
File an official complaint. If there is any physical contact, you can file a complaint with the police department.
Contact a lawyer. If still no one is taking action to stop this behavior, then you may want to look into getting some legal advice.
Notify the press. A reporter might take on the story.
Don't know how helpful this is but the answer for us after beating our heads against a stone wall in this area for three years was to take the kids out and homeschool them. If that is an option you want to consider, I would highly recommend it. We keep the kids involved socially with youth organizations and church youth group and they have learned to thrive and grow academically and socially exponentially more than they were ever given the opportunity to do in public schools because the administration is more afraid of the bully's parents suing the school for cracking down to hard than they are of the good kids' parents doing the same for not protecting their kids. Sad but true in our experience. Total lack of accountabililty or consequences for the troublemakers and even blaming the good kids for causing the problem when they are the ones taking the heat alone because no one will believe them or help them. Out of that unrelenting and life draining oven, their natural talents blossomed and their self-confidence shot through the roof and now they can deal with the occasional tough guy without crumpling...they are empowered to handle difficult social situations better because they now know that someone has their back if it gets out of control and that they actually have the skills to handle much of the guff on their own without having to be intimidated or fearful. I know not everyone has the desire or capability to homeschool but it is an option for those who can consider it.
Please go to my website as I have a free report there on Cyberbullying and what to do but the tips will help you for this as well.
That is the next step. What answer are you getting from the school and the police?