I'm not sure if Charlie even knows his name, because I have so many nicknames for him that I use on a daily basis!
I think what he responds to is the intonation I use when I talk to him. Sometimes he will run right up to me when I talk to him, and other times (usually) he will look at me and even acknowledge that I am talking to him by meowing, but will not come over. It all depends on his mood.