Interesting question! This is certainly a key point in the text. Simon specifically says, "You'll get back to where you came from," instead of saying, "we'll get back"--seems rather morose for a young boy, doesn't it? What Simon is saying is that he knows that Ralph is a strong boy and that he will make it back--you have to remember that Ralph is nervous about going to hunt at this point in the text and worries about the beast in the woods. Simon is a very empathetic person and is, on some level, encouraging Ralph. However, you have to admit that there is a bittersweet note to the text as Simon doesn't believe that he will be making it back.
At the beginning of Chapter 7, Ralph and the boys take a rest from their expedition to relax and eat fruit. While the other boys are eating, Ralph dreams of life back home and wonders if he will ever get off of the island. As Ralph peers out towards the ocean, Simon tells him, "You’ll get back to where you came from" (Golding 111). Simon's comment is significant because he is essentially foreshadowing Ralph's future. Simon knows that Ralph will make it home and return to civilization. Throughout the novel, Simon is viewed as a symbolic Christ figure. He has a deeper understanding regarding the nature of the "beast," is a magnanimous individual, and has a sense of future events. Simon's encouraging words give Ralph hope and provide him with a brief sense of peace. Later on in the novel, Simon is murdered, and Ralph narrowly escapes Jack and his band of savages. Simon's comment about Ralph making it home is accurate because Ralph is eventually rescued by the British Royal Navy.