Pusheed - think The Wicker Man!
There are lots of these wrong thread posts lately, is this a campaign to see who can make the most?
OP, we are not saying he should have to suck it up at all. I tell my kids to be proud of their accent and if someone picks on them, just to say "Yes, I'm from up North so I'd be very very afraid if I were you!" I know that kids long to fit however so I don't mind when they adopt the southern accent at school.
If it's a one-off remark I tell them to just ignore it and walk away, or perhaps to say "pathetic!" to them. But if it becomes a recurring thing then go to the teacher and ask that something be done.
At 10 he'll soon be going to secondary school and at that age I'd want to give them the confidence they need to resolve problems like this by themselves. So my advice would be to give him the tools to tackle this by himself and if that fails, see the teacher. But don't mention racism because it's not, it's kids teasing each other and being hurtful, same as if they called a ginger kid.