Why is there no "a" before "little guy"?
"Little guy" is being used as a noun here and a placeholder for a name. Just as you would not place an article in front of a name, you do not need it in front of "little guy" as it is not an adjective in this context. "Little guy" is a common term of endearment for young males. Ex: Hey there little guy. Are you hungry? Ex: What's the matter, little guy?