O.K., I posit the idea that if a general term such as "buddy", "squirt", "stretch", "wiggles", etc, has devolved to the point that a specific indidual is known in general by that noun, it is then a proper name and should be capitalized. If it is just a replacement term (often derisive) of a person, then it shouldn't be. "Hey, x-ray eyes, come over here and lend me some of your super strength."
I did know a very tall fellow who had been called "Stretch" since High-School. We called him that and I would then capitalize the word "Stretch." Now where life became interesting was that since he was a Navy captain, how you addressed him reflected both your role and his role in the conversations at hand. At work he was Captain, or Captain Youngblood. In the bar, he was Stretch. Or if it was just the two of you in a casual conversation, it was Stretch.
Artemis
Memory just struck. I called my son "tiger" when he was little, but it never was his name. If I ever wrote it down, I would caitalize it. It was his nickname.
