The answer to your question lies in recognizing Simon as a Christ-figure in the novel. Simon realizes that he himself will never leave the island. He understands that he is on the island to identify the evil among them, which he does when he sees the "beastie" for what it really is--not a dead parachutist on the mountain, but instead the evil that abides in each of them. The symbology of Simon as a Christ-figure can be documented in his gentle dealings with the young children, with the way he goes into the "wilderness" (his hiding place) and is "tempted" by the "lord of the flies" (which translates to the word "Beelzebub"), and with the way he is killed when he was merely trying to proclaim the truth to the other boys. A part of Simon realizes early on that he is not meant to escape this island, but instead to serve as a moral compass for both Ralph and Jack as each struggles with the decision the island poses--civilization or savagery. By telling Ralph that Ralph will survive, he is in a sense saying that civilization, too, will survive, but not without sacrifice.