Answer
The male co-star of Leslie Mann in the comedic film "Big Daddy" is Adam Sandler. The plot follows Sandler's character Sonny Koufax, a man-child who adopts a 5-year-old boy, Julian (played by Cole and Dylan Sprouse), to prove himself to his girlfriend Vanessa (played by Kristy Swanson). Throughout the movie, Sonny and Julian form a profound and heartwarming bond with support from his friends, including Mann's character Corinne.