What is Simon trying to tell the boys when he stumbles out onto the beach?

That the beast on top of the mountain is not real.
That they are being rescued.
That he is sick.
That he wants to be the leader.

That he has discovered the true nature of the "beast" and that it is within each of them and not an external creature on the mountain.

Simon is trying to tell the boys that the beast on top of the mountain is not real.