PuzzledMum - stick with it! It's early days and probably a lot of those budding alpha males who are used to being top dog in their cosy, nurturing primary schools are flexing their muscles a bit to establish themselves in their new environment. It'll probably settle down a bit shortly. Name-calling / ribbing / winding people up does seem to be something that certain groups of boys/men enjoy. I've come across blokes who even in adulthood refer to themselves as Stinky or Malfunction, as that's what they were called at school (and actually secretly like it now.)
It's not always meant in a bad way. Let's face it in an all-boys school, there's unlikely to be a lot of sitting down and talking about people's feelings, is there?
It's good that you've told the school, so they can keep an eye on it. Have you told your son to try not to rise to the bait? Obviously if people see he's upset, it'll only encourage the perpetrators. I would encourage him to join some extra-curricular clubs at school too. That will hopefully widen his circle of friends. In many boys' schools joining in at sports is particularly highly valued, so if he can join the rugby club or whatever that might help. He doesn't necessarily need to be good at it - just participating will help.
My son went through similar experiences when he started at GS. He would regularly tell me of a group of boys who would throw his belongings around, put him head-first in the bin etc. Now at the start of Y8 those same boys are some of his closest friends.
They've all calmed down. There's still some ribbing, but done in a generally good-natured way. I hope things work out for you too.