> How do I convince him that this peer pressure is unacceptable and that iPhones are not automatically 'better' just because they're marketed as a premium brand are are nothing more than expensive jewelry?

Whether the phone is any good or not is besides the point. The point is that he's being bullied for it. Perhaps you could try raising the issue with the kids' parents?

I'd also suggest that there's an extent to which taking a principled stance on these issues isn't worthwhile. Such bullying can be really damaging to kids (as I know from my personal experience as a child), and hepling them to fit in can be the best way of protecting them from this stuff. That doesn't mean you accept the line of reasoning they are hearing. You should still provide a strong rebuttal to that. But it might mean that you support them to do things that wouldn't otherwise be rational simply because they want to fit in.

EDIT: I also agree with another commenter who says "Focus on his self confidence". If he's confident in himself then he'll be able to navigate the social situation much better.