What is a good form of address for a male friend who is too old to be 小, too young to be 老, and too humble to be called 大?
if he is older than you are, then 柯兄 (柯老兄)
if he is younger, then 老弟 (柯老弟)
if he is of the same age, and his first name consists 2 chinese characters, then first name
if he is of the same age, and his first name consists 1 chinese character, then full name (calling someone by their full name is not rude if they are the same age or younger in China)


EDIT: I'm from Northern China and we don't use 阿X as often. that's more of a southern convention.

Note: if his first name is one character, then calling him by his first name sounds 肉麻。 It is actually worse if you are of the same gender. then that just sounds wrong! :P