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? I have a male "friend", last name 柯 (kē) who is over 30.  From ages 18-30 or so, he went by 小柯, but that seems less and less appropriate as he gets older.  Always going by his full name seems overly long and formal, he doesn't like "大", and he's not old enough for 老 (yet).

My "friend" was thinking about going by 阿柯 (ā kē) and having friends introduce him this way.  He thought the main benefit would be that people both younger and older could use this friendlier, less intimidating and formal means of address.  My "friend" isn't a native speaker, so he's not sure if there are negative connotations.  

So would 阿 [LAST NAME] be the best form of address for these conditions or if not, what would a better one be?

[EDIT] I should have originally specified that he himself is looking for something to be called "around the office", that is, both by people senior and junior to him.  His boss calling him 老柯 or 柯哥 doesn't seem quite appropriate.  It does seem that 哥 is the most popular "among friends" answer.
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