Your son NEEDS a complete evaluation to get to the bottom of WHY he was in a group that wanted to kill someone. That is NOT normal, as I am sure you know. I would be FURIOUS that my son was the only child punished. Furious enough to ask the Superintendent why only one of the children who were in this group was punished, and why he is now being ostracized??? Ostracizing a child is NOT going to make him feel all warm and fuzzy about his classmates and teachers. DUH.
I would try to speak to Eric's mom to let her know I was sorry, was addressing this with my son as a very serious issue, and that I am very concerned about her son's safety because ONLY ONEchild in the entire group was given any punishment and now the other students are targetting your son, so clearly the other children involved did NOT learn anything about how wrong and terrible their actions/intentions were. If my son was Eric and I learned that only ONE of the big group of kids who ALL joined together to kill my son was punished I would be outraged, furious and very scared for my child's safety.
If you address it from that angle, instead of from the "why was my child the scapegoat" angle, you are far more likely to get something done to address this.
What other problems is your son having at school? Why was he involved in a group that was formed with the purpose of killing someone? What, exactly, did Eric do to warrant this? Learning this why will give you the key to helping your son learn that this is incredibly inappropriate.
His statement about tolerance could well be a sign of Aspergers. It also could be true. How much teasing does he get at school? Why is it allowed to happen? It sounds like it is bullying, at least to your son. The true measure of if something is bullying or not is how the target feels. Did your son join this group because Eric bullies him? How can that be addressed?
You school is being outrageous and idiotic. I would be very unhappy also. Does your son have an IEP? If not, he needs to be fully evaluated and then given an IEP. Go to the Sp Ed archives for a letter that you can send to start the ball rolling for an IEP. It puts legal protections into place for your son. To do this it MUST be sent via certified mail, return receipt requested. Otherwise you have no proof it was ever sent and the school is free to ignore your verbal or otherwise not officially received request. Right now schools do NOT want to add accommodations and supports for students because the budget problems. That is NOT YOUR PROBLEM and if they even say anything about the cost it violates Federal Law. Schools DO try to ignore or postpone requests for IEPS because they have to do testing and it costs. They also have to do more for the kids and that costs and complicates things for them. Or that is their rationale.
I am sorry your school is so messed up.