Simon is the boy who provides Ralph with emotional encouragement, in the same way Piggy provides him with intellectual support. In this scene in chapter 7, Simon senses that Ralph is worried about never being rescued because of the huge ocean that separates them from their home. As a very sensitive and empathic person, Simon has read Ralph's thoughts. More than that, though, he has sized up the other boys on the island and evaluated their situation from a perspective no one else has. He is the only one who, when he thinks of the beast, sees with "his inward sight the picture of a human at once heroic and sick." He suggests about the beast in chapter 5, "maybe it's only us." He was right there when Jack beat Piggy and broke his glasses, and in a conversation with Piggy and Ralph where Piggy warned Ralph of Jack's hatred, Simon concurred. When Simon says "you"ll" get back to where you came from, he may have a premonition that he himself won't. This foreshadows Simon's vision and death in chapter 8 and his death in chapter 9.

Ralph is the one who is trying to keep his wits about him. The boys are stranded on a deserted island and he has high hopes they will be rescued. Simon is very in touch with the nature around him and his quite intuitive about things. He knows that some of the boys will not make it off the island.

In chapter 7 we see that Ralph sees the ocean as a barrier to them being rescued. He sees it as a physical wall to anyone coming to rescue them. Simon tells Ralph not to worry because he knows that Ralph will be rescued. It does give Ralph a little bit of hope. In chapter 7 we also see some of the boys becoming more and more out of touch with reality. Ralph is losing any control he may have had on the boys. Piggy is basically the only one who is truly on his side. I also think Simon is talking about the mental stability of some of the boys. Yes some of them are rescued, but will they ever be able to "leave" the island and the events that happened there?

Most of the boys have gone mad and although they will be rescued, the horrible things they did on the island will follow them forever. Some of the boys will never be able to really leave the island. 

Simon is an intuitive and sensitive young man.  He easily sees that Ralph is nervous about the hunting trip and what the boys might find on the hill.  He singles Ralph out because he respects Ralph, feeling that Ralph is trying to be a fair and compassionate leader,  unlike Jack, who just wants power.

In chapter seven, Ralph is down and out. He fears they will never be rescued. He wanders down the beach. He is in deep thought. He thinks bout how vast the ocean is. He realizes that their chance for rescue is limited:

Unconsciously, he is tense, gripping a rock, arching his back, mouth strained open.

That is when Simon appears. He is encouraging. He tells Ralph that he'll get back. Simon is reassuring Ralph that he will get rescued. Ralph makes a negative comment. Simon again assures Ralph that he will get back to the life he once knew:

Simon appears at his elbow. “You’ll get back,” he says, insightfully knowing what Ralph is thinking from observing his posture. Ralph calls him batty and Simon repeats that he just thinks he’ll get back all right.

Simon always knows what to say. He is very perceptive. He can tell that Ralph is down. Simon tries his best to lift his spirits. He is certain that they will be rescued. He conveys this message, hoping to cheer Ralph. Simon must have been encouraging for the two boys suddenly smile at each other.

The two boys suddenly smile at each other.

Simon knows what to say to make Ralph smile. He knows that Ralph needs someone to be supportive. Simon is indeed a caring, sensitive individual who makes Ralph smile. Ralph needed to hear what Simon had to say.