Whether the pig's head says anything at all to Simon is questionable. What has happened is that Simon, the intuitive member of the group, has suspected for some time that the boys have been degenerating into savages.
After Simon retreats to his solitary place in the jungle, Piggy dismisses him as "cracked." But, Simon is simply disturbed by the insight at which he has arrived. When he perceives the head in his hiding spot, Simon imagines that head speaks to him when the knowledge is really in his heart.
The half-shut eyes were dim with the infinite cynicism of adult life. They assured Simon that everything was a bad business.
" I know that." ....Simon had spoken aloud.
....Run away, said the head silently, go back to the others. It was a joke really--why should you bother? You were just wrong, that's all. A little headache, something you ate, perhaps. Go back, child, said the head silently.
Simon realizes that he sees evil incarnate. The "Lord of the Flies [Beelzebub, the devil] hung on his stick and grinned." For Simon, "what was real seemed illusive and without definition." Intuitively, Simon apprehends that the evil is in the boys themselves: "and his gaze was held by that ancient, inescapable recognition." Then, Simon "hears" the Lord of the Flies" call him a silly little boy who the others think is "batty." Simon imagines that the pig's head taunts him. His mouth is dry and he feels faint, but he says, "Pig's head on a stick." However, Simon believes that the pig's head "replies,"
"Fancy thinking the Beast was something you could hunt and kill!"...For a moment or two the forest and all the other dimly appreciated places echoed with the parody of laughter. "You knew, didn't you? I'm part of you? Close, close, close! I'm the reason why it's no go? Why things are what they are?"
Simon hears it talk to him in the voice of a schoolmaster, scolding him and asking, "do you think you know better than I do?" It warns Simon that it is going to get angry and tells him he is not wanted because he will get in the way of their "fun."
"--Or else, " said the Lord of the Flies, "we shall do you. "
Simon collapses from a seizure as he imagines falling into the beast's mouth. Here in this chapter is the moment of truth for Simon, and a very telling one, at that. With the fire having been stolen, anarchy reigns and evil sets upon the island.
At the end of Chapter 8, the Lord of the Flies (the severed head of the pig) says a bunch of stuff to Simon as Simon is hanging out in his secret place, which is where the head has been placed.
I think that the most important thing that the pig's head says to Simon has to do with what the Beast that they have all been worried about is. The head tells Simon that the Beast is not some living animal that is out there. It tells Simon (and it believes Simon already knows this) that the Beast is something that is within the boys already. The Beast is the reason that things on the island are as messed up as they are.