It's probably because he is sensitive and emotional that he gets bullied, as other kids seem to be able to smell that it will have an effect on such kids. The other kids are probably falling in with this ring-leader because they would rather it was someone else them themselves that gets this unpleasant attention.
Why not try the school first, find out what the teachers know and what they can do to help, and if that doesn't work, perhaps take a deep breath and try the mum? Lots of teachers take bullying seriously and it's easier to deal with at 6 than 16.
A big hug for you, and an even bigger one for you LO.