This a good question and a number of factors points in the direction that Simon is a Christ figure. Let me give you a few of them.
First, Simon is consistently portrayed as a selfless and generous boy. While the others are doing their own thing and playing around, there is a seriousness about Simon. He is helping Ralph build huts. This selfless quality is like Christ. The gospel of Mark says:
"The son of man has come not to be served, but to serve and give his life as a ransom for many."
Second, Simon has a spiritual quality about him. In this way, he is very different than others. For example, he is marked by faith. So, he reassures Ralph that all will be well, but he says that he might die.
Finally, Simon does die, like Christ, and his body is mysteriously disappears.
"lifted a fraction of an inch from the sand and a bubble of air escaped from the mouth with a wet plop...Simon's dead body moved out toward the open sea."