The title refers to the 1940 novel 'For Whom the Bell Tolls' by Ernest Hemingway about a group of anti-fascist guerrillas during the Spanish Civil War; the novel's title, in turn, was taken from John Donne's 1624 work "Devotions upon Emergent Occasions" ("... never send to know for whom the bell tolls; it tolls for thee").
As the boys run in the house to see 'Uncle Stu,' Mark sees him and, as he jumps into Uncle Stu's arms (Christopher McDonald's character), says, "Mikey." Ultimately, Taran Noah Smiths' character is named Mark. Closed captioning confirms, he says, Markey.