I think it works better all round if kids can sort out these issues among themselves, but I'm not sure what to suggest DS do. I know he has told the other boy to stop, and he has told him no (those are the snippets he's told me).
I don't know whether to tell him to ask a teacher for help because some teachers would consider that tattling and that kind of reaction makes things worse for the child being tormented.
I'm trying to think of ways to empower him, but I'm a bit stumped. Thinking back I remember my dad advising me to punch a classmate on the nose, and my mum stopping a 6 year old on the way to school one day with a look that could curdle milk and the advise to leave me alone. I'm not sure I'll be asking either of them for advice!