The boy is Robert. This incident occurs immediately after the boys had returned from an unsuccessful hunt. They had left Piggy at the shelters with the littluns. Their endeavour was unsuccessful because the boars they found escaped. Ralph had actually managed to injure one of the boars when he launched his spear into its snout. He was very proud of his achievement and sought recognition.
At this point, Robert turned to snarl at Ralph as he illustrated how he had thrown his spear. Ralph entered into the play and soon all the other boys were jabbing their sticks at Robert. Jack ordered that they form a circle and soon the boys were attacking Robert who at first cried out in mock terror, but soon screamed in real pain, for the boys were hurting him.
The boys carried him off and rhythmically started chanting that they should kill the pig. Robert was in a real frenzy, crying and screaming until finally Jack indicated a stop. Robert had been hurt and was snivelling in pain and shock.
The significance of this scene is that, firstly, Ralph discovers the savage within him. He experiences an almost uncontrollable desire to hurt Robert and jabs his stick at him. When Robert is carried off Ralph was also fighting to get close:
The desire to squeeze and hurt was over-mastering.
In addition, this event displays the fact that even the best and most controlled persons have an innate savagery which would come to the fore in the right circumstances. The event is also clearly indicates the instinctive brutality and ferociousness of the boys. They lust to hurt - it is not only about getting meat, but more about the excitement of seeing blood and the thrill of hurting a living thing. Since this desire went unfulfilled, they had to dissipate their lust in some other way and therefore turned on Roger.
Also, the incident foreshadows Simon's death on the beach. When he comes out of the forest during the downpour, the boys believe he is the beast and they all ferociously attack him, Ralph included, killing him.