Marc Mezvinsky has an impressive fan in his future father-in-law, former President Bill Clinton, who says he is "very" excited about the engagement of his daughter Chelsea to the investment banker."He is great. He is a great human being," the former president said Tuesday as he left a panel discussion at Columbia University's Mailman School of Public Health, where his daughter is a graduate student. The gathering was designed to draw attention to World AIDS Day.As for Mezvinsky, "Bill adores him. This is the son he didn't have," says a source close to the Clintons. "This is the relationship Bill didn't have with his father."Ever since Mezvinsky, 31, and Chelsea Clinton, 29, announced their engagement last week, her dad's been beaming from ear to ear – he's just that fond of the new addition to the family."Very often, when Hillary is traveling," the source says, "Chelsea and Marc and Bill go out to dinner, or they go to Chappaqua and go to the movies."Adds the source, "It's a very comfortable relationship. This is a nurturing relationship."