Ace boom coon has been used for more than 30 years as a term of endearment, like a best friend, meaning someone who is a good friend, someone you care about or someone who is a close to you or you have been knowing for a long time.
Person 1: Sees a friend walking down the hallway at school, gets excited and says "Oh, that's my Ace Boom Coon!"
Person 2: "You know him?"
Person 1: "Yeah, we go all the way back to elementary school and the hood. I been knowin' him foreva. That's my dawg."