First of all, it's a joke. "A rising young man" means someone, probably in their twenties, who is just starting their career, and is having great success.
Warren Buffett is 88 and has been CEO of Berkshire Hathaway for forty-eight years. He isn't a young man. He can't be "rising" since he is already at the top. By jokingly calling Buffett "a rising young man," Munger is really saying "Warren Buffett isn't sick or dying, and he is going to continue to be CEO."
The phrase "one" before somebody's name means "this isn't a famous person. I don't expect you to recognize the name. I've never heard of the person before." It is often a mild insult. This is part of the joke, because Warren Buffett is famous and in the business world everyone knows his name.
Here are some examples of use. In the "Superman" fictional universe, Clark Kent is a senior reporter, while Jimmy Olsen is an inexperienced young "cub" reporter. The mayor's secretary gets a telephone call.
Secretary: "I just got a call from Clark Kent at the 'Daily Planet.' Will you talk to him?"
Mayor: "Kent? Yes, of course, I know him. Put him through."
Secretary: "I just got a call from one Jimmy Olsen. He says he's a reporter at the 'Daily Planet.' Have you heard of him?"
Mayor: "Jimmy... who? Olsen? Never heard of him. He's nobody. Tell him I'm busy."
Secretary: "I just got another call from Jimmy Olsen. Will you talk to him?"
Mayor: "I've heard a more about him. He's a rising young man at the 'Daily Planet.' I'll talk to him."