This is going to sound like a cop-out answer at first, but bear with me. Simon is neither a leader nor follower AND is both at the same time.
Let me start with the idea that he is neither a leader nor a follower. He is not a leader because he doesn't actually ever lead any of the boys that are on the island. Ralph gets voted chief. Jack is appointed leader of the hunting party. Jack is probably the biggest leader of them all, because boys follow his lead throughout the novel. Ralph is losing his leadership role almost from the moment it is given to him, but he does make important decisions and lead for a while. Simon doesn't even have a little group of littluns following him. He is frequently off by himself. That's when he finds the pig's head on the stick. That's when he discovers the monster is a dead man. He is alone, and that is precisely why Simon is not a follower either. He doesn't follow Jack and hunting. He doesn't follow Jack's moral degeneration. Simon also doesn't necessarily follow Ralph and Piggy around either, because he is too often out by himself to follow anybody's lead.
Simon is about the only truly good and moral character in the book. It is innately part of him. Jack is only good when it is forced upon him. Ralph and Piggy are merely trying to reflect the society that they knew back home. Simon is genuinely a good, moral character.
Therefore, because of his inherent goodness, Simon is a leader of sorts. The other boys may find him odd, but that is because of how visible his morality is. It's unsettling to the other boys. Had the others hung around Simon more and/or had Simon lived for the entire book, his goodness may have begun to influence those around him. That would have made him into a leader. I've read an analysis or two that compare Simon to a Christ figure. Regardless of your thoughts on Christ, he was a leader, not because of his physicality or ruling position, but because of his goodness to everybody around him. Simon is exactly that way with others, so he could be a great leader.
The main issue with that argument, though, is that Simon acts too much like a follower to truly lead through his goodness. He spends too much time alone; therefore, he is never putting himself in a position to be a moral leader. And if the only option other than leader is follower, then Simon has to be a follower.