I see in my above post that I left out the quote!  I have one for you: 

“Here, invisible yet strong, was the taboo of the old life. Round the squatting child was the protection of parents and school and policemen and the law. Roger’s arm was conditioned by a civilization that knew nothing of him and was in ruins.” (Chapter 4)

As I mentioned in my post above, Roger fears the condemnation of his peers if he actually chunked a rock at the boy.  He's still functioning within the laws of "civilization;"  the intangible circle of protection around Henry are the lingering vestiges of civilization; Roger still feels it keenly and throws to miss.  Only later in the novel when Roger adjusts to the wildness of the island coupled by the savagery of the hunters does he throw off those old taboos of "parents and school and policemen."  When Roger loses his inhibitions, his violence escalates, ulitmately resulting in the calculated murder of Piggy.

Early on inLord of the Flies, Roger throws stones at young Henry but purposefully misses.  In this scene of the novel, Roger tests the boundaries of accepted behavior; he pushes the limits to test and gauge the other boys' reactions.  He chooses to miss, because he feels insecure about their reactions and does not want to open himself up to criticism.  Roger is a bully at heart, cowardly targeting boys who cannot defend themselves like Piggy and Henry. 

Roger's throwing of the stones is like a test-run for later in the novel; he enjoys the feel of superiority it gives him over the weaker boys.  Later when the savagery increases among Jack and the hunters, Roger will feel comfortable to act even more brutally and does; this time, he will not miss, and Piggy pays the price.