My experience with people like this is that they eventually grow out of it. I spent my entire high school years trying to expand my musical horizons and find some really cool stuff, but everyone in my social group was content to sit around listening to really shitty metalcore. I've had a few of those friends come back to me and say how awesome Slayer and Megadeth are and how they wish they could have gotten into them sooner.
Of course, I had a really narrow musical taste when I was in middle school. I only listened to Metallica, and maybe a few other bands like Megadeth, Pantera, Slipknot, Anthrax, whatever. It wasn't until I got sick of those bands that I began to explore further. It really depends on how early you get into it, and how long you've known the bands. Eventually, Metallica WILL get boring, and if you're lucky enough to tolerate him until then, you can be the cool dude who knows all the awesome bands.
tl;dr: Let him listen to his boring metal until it gets boring to him, and then go nuts.