Most Helpful Guy
"He doesn't want a serious relationship, so we're just kinda friends," that should be enough. He sees you as his friend, and not a romantic partner, I guess. Most guys call their friends "bro" or "dude", so that's what he calls you, even if you both like each other
Most Helpful Girl
He doesn't want a serious relationship and calls you bro. That definitely sounds like he just wants to remain friends and nothing more. I've never found the term "bro" a flirtatious term. I don't know, he may just be different towards the term though.