I feel for your situation. My son (13) has Aspergers and we moved him from his school to a new one. We also made sure the school was aware of and on board with his diagnosis and his self-esteem issues. He now has a small core of friends, he is doing well in classes and he no longer cries at night because he feels 'different'. Please talk to the school and ask them to support him more including ensuring he is not picked on. And please think about moving schools. It is not always the answer but I gave my child all the advice you give your son and it doesn't work if the kids themselves are relentlessly picking on him. But you are so great to be so supportive and believe me that means so much to them. Does the school have a counsellor he can talk to or chaplain? Or is the principal or vice-principal open to discussion about this? You can advocate for him and if they won't or can be helpful, do look at alternatives. Is he interested in something like Karate or Judo? theses are great activities for helping kids, especially young men, feel confident and my so did classes for about a year and really loved it. I hope you will keep posting as I understand your pain and I wish you all the best.
I'm so sorry your son is going through this, it must be really horrible that he is being so relentlessly bullied by these classmates he thought were his friends. And I am really feeling for you being in this situation, watching someone else being bullied is so stressful and can leave us feeling quite helpless - but you are clearly doing a wonderful job at supporting him however you can.
That is great that he gets along so well with his teachers that can be so helpful, @Hopesprings123 is right that getting the school involved in bullying is often a great step, could you two talk to some of his teachers together? Schools can be very helpful in these situations and they have a duty of care to look out for student wellbeing.
We've got a great piece of content which I will link here with some information about teenagers and bullying. It has a great video at the bottom from psychologist Clare Rowe who has some good tips oh helping teens who are being bullied.
You are certainly right that we can't force anyone to engage in support from a counsellor, but sometimes over time young people can warm up to the idea a bit more. Do you think he might be more keen to engage in online supports? Let us know if you want to chat about this more.
It is also so lovely to hear that you are focused on giving him so much love and support - that is truly the most important thing and he is lucky to have you in his corner