sorry, i think it's ridiculous to label someone not wanting to be your son's friend as 'bulling'. bullying is serious and needs to be addressed. the tendency to label 'anything my kid doesn't like' as 'bullying' is an issue. it indicates in my mind a degree of parental laziness. instead of working with one's child on how to be strong and resilient and discerning, it's so much easier just to make it every other kid's fault.
so the 'bulling' that's not 'excepted' in your realm is not the problem. the problem is that you're not actually wanting to do what your (sensible) question asks, which is how to take the power out of the threat.
instead of running to the school and expecting them to police all the other kids, i'd keep this one very light. i'd listen carefully and quietly while he told me how he felt about this other boy. i'd ask leading questions like 'how do you behave to other kids with whom you'd like to be friends?' then 'and is this how this boy was acting? no? so is he really a friend?'
and i'm very put off by you insisting that YOUR kid is being bullied, but being just fine with all the other boys excluding the 'bull' to the point that he was in tears. so your kid can be a 'bull' and it's great when he makes another kid cry?
now, i actually think that the exclusion is a perfect response. the other kids are doing exactly what the wolfpacks have always done- hazed unacceptable behavior and handled it in the herd-or-exclude fashion that humans have always done. the social media 'unfriending' is just the next step in the ongoing paradigm.
so the kids are handling it, your son is handling it, but you still feel the need to micromanage all the behaviors.
THAT is what i see as the problem.
khairete
S.