Jack Harlow has known his life's purpose since he was a 12-year-old sixth grader rhyming into a microphone from the video game Guitar Hero: He was going to be rapper. His first solo mixtape was a hot-seller in the halls of his Kentucky middle school. Now, the 24-year-old Grammy-nominated star is selling-out venues across the country and around the world. In this week's "Sunday Sitdown," Willie Geist gets together with the rapper in between his two Boston shows, where fans were packed shoulder-to-shoulder singing along to every word.