If he is younger than you or have the same age as you you call him by his name and add 아 or 야 as Mister Peter said in his comment. for close friends: - if you are a girl you call an older boy "oppa" (오빠), and older girl "eonni" (언니) - if you are a boy you call an older boy 'hyung" (형), and older girl "noona" (누나) check this article for more information: http://seoulistic.com/korean-culture/what-it-means-to-be-oppa-unnie-hyung-noona-older-in-korea/

same age 'as' me

I live in Korea, and from what I've seen, you can just take someone's name and add 아 if it ends in a consonant or 야 if it ends in a vowel to call their name. If you're talking about them, just use their name normally. Just be sure the person actually is the same age as you. There's a small possibility that I'm wrong, but even if I am, I know for a fact that someone who is your age still wouldn't be offended at all considering that you're a 외국인.

친구 . Chingu