In a more symbolic sense, the pig's head represents evil incarnate, temptation. We know that from Golding's notes on the novel in which he equates "Lord of the Flies" to Beelzebub. On the other hand Simon has striking similarities to Jesus. He is kind, compassionate, intuitive, prophetic, and messianic. The pig's head acts as his tempter, equivalent to Satan's temptation of Jesus in the wilderness. What Simon is being asked to do is "have fun on the island." This phrase is most often used by Jack, and to him "having fun" means hunting and killing. Almost every other boy on the island with the exception of Piggy enjoys the savage nature of the hunt. Even Ralph enjoys participating in the boar hunt, and later engages in the pig-hunt reenactment, which almost goes too far--Robert acting as the pig is almost hurt by the boys surrounding him.
Simon is asked to join in the fun or savagery. In order to survive on the island, he must change. The consequence for remaining civilized is death. As the pig's head tells Simon, the boys "will do him" if he does not "play."
It is not Simon's nature to change, to become savage, to hurt, or to kill, and he resist this temptation by falling into a fit. When he comes to, he goes to tell the boys the true nature of the beast. This action leads to the fulfillment of the pig's head's prophecy. The boys do indeed "do him."