OxfordDictionaries.com defines epilepsy as "a neurological disorder marked by sudden recurrent episodes of sensory disturbance, loss of consciousness, or convulsions, associated with abnormal electrical activity in the brain."
The first reference to Simon's condition is made in chapter one, with the arrival of Jack Merridew and the choirboys:
Then one of the boys flopped on his face in the sand and the line broke up. They heaved the fallen boy to the platform and let him lie. Merridew, his eyes staring, made the best of a bad job. “All right then. Sit down. Let him alone.” “But Merridew.” “He’s always throwing a faint,” said Merridew. “He did in Gib.; and Addis; and at matins over the precentor.”
We know that this refers to Simon because we later read:
... the choir boy who had fainted sat up against a palm trunk, smiled pallidly at Ralph and said that his name was Simon.
Simon's condition patently differentiates him from the others, and Ralph at one point remarks: ‘“He’s queer. He’s funny.” Jack nodded, as much for the sake of agreeing as anything ...’ Ralph also calls him 'batty.'
The second, and more significant, reference to Simon experiencing an epileptic episode is found in chapter eight, as per your question. In this instance, the text suggests a clear link between Simon's hallucinatory state and the onset of an attack. The heightened electrical activity in his brain may be what causes him to imagine that the sow's head on a stick (The Lord of the Flies) is conversing with him.
Simon’s head wobbled. His eyes were half closed as though he were imitating the obscene thing on the stick. He knew that one of his times was coming on. The Lord of the Flies was expanding like a balloon. “This is ridiculous. You know perfectly well you’ll only meet me down there—so don’t try to escape!” Simon’s body was arched and stiff. The Lord of the Flies spoke in the voice of a schoolmaster.
Simon gradually loses consciousness.
Simon found he was looking into a vast mouth. There was blackness within, a blackness that spread. “—Or else,” said the Lord of the Flies, “we shall do you? See? Jack and Roger and Maurice and Robert and Bill and Piggy and Ralph. Do you. See?” Simon was inside the mouth. He fell down and lost consciousness.
This experience precedes Simon's discovery that what the boys believed to be the beast was actually a dead parachutist. He resolves to inform the others about his discovery.
The beast was harmless and horrible; and the news must reach the others as soon as possible.
It is unfortunate and tragically ironic that he never gets the chance to deliver his message, for he is killed by the frenzied boys when he appears from the forest and is mistaken for the beast.