How to stop a nickname?

[Thematic]

My 11 year old son has recently started secondary. A nickname he had at junior school has followed him to secondary- surprising as there aren’t a lot of his old friends at this school. It’s quite a fond nickname, not bullying or teasing or anything but I think he had hoped to leave it behind him. How on earth can he stop this without making it a big deal? He’s rightly pointed out that if he tries to tell people he doesn’t like it, that’ll just make them use it more- and he’s right, boys pick up on any sign of weakness. It seems like doing anything will just turn it from a fond nickname to bullying. Any ideas how he can nip it in the bud now whilst he’s still new and before it becomes fixed?

Just don't respond to it?
If people don't get an answer unless using his proper name they might get the message.

Or is there something else he would prefer to be called - he could try introducing that.

Choose a different nickname/abbreviation.

He's less likely to get teased if he says "I'd like to be called Billy" than if he says "I don't want to be called Bobo".

But it may not work.
My cousin had a friend who went to nursery as BJ - short for Baby Jonny, as dad was also Jonny.
The parents (and him) tried several times to change it, but people just forgot. He's in his 40s and still called BJ now.

I agree with PP that if he can offer an alternative nickname it would be better received. So, saying something like “my friends call me Robbie” rather than don’t call me Bob.

If it's innocuous, maybe he needs to just learn to live with it.