The term "idol" only occurs once in the entire book, in Chapter 9. While the boar head on a stick serves as the physical representation of a primal superstition, it is never called an "idol," instead called a "beast" or The Lord of the Flies, as it attracts flies through decomposition. An idol is a physical representation of a supernatural being, such as a deity, and so the boar head should have been the obvious reference. However, that is not the case; instead, the word is used in a different context.
Before the party had started a great log had been dragged into the center of the lawn and Jack, painted and garlanded, sat there like an idol. There were piles of meat on green leaves near him, and fruit, and coconut shells full of drink.
(Golding, Lord of the Flies, staff.bcc.edu)
Jack and Ralph have spent much of the book trying to outdo each other as leader of the boys; at this point, they have split into two rough groups, and despite Ralph's greater rationality, Jack's blustering "bravery" attracts the other boys who are seeking childish adventure. In this scene, Jack is set up as a classic tribal chief, representing the power and strength of the tribe. He has tributes, or sacrifices, of meat and fruits around him, and he sits alone in a position of power. In this sense, Jack is the idol to which the other boys defer, since he has established his power through brazen strength. Since the boar head only appears to the delirious Simon, Jack is the person to which the term "idol" refers.