Since readers default to Caucasian, you have to say--but you don't have to be the narrator making an announcement. You can have other characters interact with your Korean guy and make remarks, ask questions, whatever.
I'd been in Heidi's class since sixth grade and she'd never spoken directly to me, but she walked up to us and interrupted. "Daniel, can you help me?" She tossed her long hair over her shoulder.
"Uh, sure, I guess."
"It's calculus. I know you're good at math."
My friend shared a look with me. "I am?"
"Sure, all of you guys are good at math."
"Us guys, people who went to East Middle School?"
"No, Asian guys."
"You're thinking of Japanese guys. I'm from Kansas City."
She laughed uncertainly. "No, your people."
"Oh. My parents are from Philadelphia. My grandparents, they're from Korea. No special talent for math."
"Not really. I got a B in algebra--just barely."
This makes Heidi and Daniel be the ones who tell the reader.