Ralph advises Piggy to write a letter to his Auntie, which definitely makes the reader wonder if Ralph himself ever composed imaginary letters to his father. If he did, Ralph would most certainly express his desire to be rescued from the island. He would probably try to describe the geography of the island in hopes that his father could narrow down the whereabouts of the boys' location.
Ralph would probably also ask his father for advice. In chapter five he despairs:
"If only they could get a message to us," cried Ralph desperately. "If only they could send us something grownup...a sign or something" (94).
From the beginning of the novel, the reader knows that Ralph's father is a commander in the navy. Now, on the island, Ralph must lead the other boys. He could certainly use some advice on leadership. He struggles with enforcing his rules, like how to keep the signal fire lit. Ralph would want to know some pointers on how to deal with dissent in the group and his father's take on how he should deal with Jack especially.
In the end of the letter, I believe that Ralph would search for reassurance from his father--to know that his father is actively looking for him and that he cares about Ralph.