how is simon having a discussion with the sow's head? what is the conversation between simon and the lord of the flies about?

The discussion is metaphorical. Simon is addressing his dark side. Simon is Golding's Christ figure. He faces his own daemons. The daemons or in Simon's case Beelzebub (Lord of the Flies), admonishes him for wanting to save the boys from themselves. Both physically and emotionally.