You can't predict all the tricks people will play on your child, so don't beat yourself up over it. But now that you know, you need to run scenarios and teach him both how to stand up for himself and how not to fall for manipulative behavior. Maybe other members here can help you brainstorm more scenarios to script and practice; I'm afraid I'm drawing a blank right now; but the bigger the data base, the better.
I would also arrange a meeting with his counselor at school so that the school knows he's susceptible to this type of bullying. Yes, he was bullied in this situation, even if the form was that he was talked into bullying someone else.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).