Leper is interested in bugs, nature, and not necessarily being involved in Blitzball, except on the sidelines. He's a typical weakling or even a nerd. He seems to be anything but army-ready.
Leper changes his heart when he watched the ski troops video the army recruiter presented. This is significant because it demonstrates the power of persuasion. He chose the army naively because he thought it was going to be fun. War isn't fun. It also demonstrated that the students who generally lead or make the first attempts (Finny or Brinker) in this case didn't.
Leper's change of heart after seeing the film of the ski troops (ironically not American soldiers, at all), is significant for another reason, as well. It emphasizes the private fear that all the boys feel as the year wears on, in terms of their own participation in the war. Leper enlists after seeing the film because, as Gene says, Leper had found a "friendly face" to the war. He thought he had found a way he could stand up to and survive what was coming for all of them. As Devon's "first recruit," Leper was the first to reveal just how afraid he really was by acting in a way that he thinks will stave off destruction. He was wrong, of course, with tragic results.
This fear of not being able to meet the challenge of military service, the fear of failing as well as dying, was seen also in Brinker. He dressed like a soldier for a time and sometimes wrote sarcastic poems about the war as a means of covering up his fear. He made plan after plan to enlist, but never did. He thought of service in the Coast Guard as a refuge, a plan his father rejected as cowardly. Like Brinker's various strategies, Leper's change of heart was an attempt to prevent or control the uncontrollable.
Just to continue where Missy575 left off, Leper is seen cross country skiing by Gene and others while they are on their way to shovel snow. When he sees the film and sees that skiing is involved, he can't wait to join, apparently. The skiing does it for him, I'd suggest. The idea of the war being a ski trip is an illusion, of course, and the illusion fails quickly after Leper joins up.
His change of heart is also significant because of what it leads to. Leper's mental breakdown destroys an illusion that the other boys at Devon cling to, that Leper is out kicking butt and becoming famous in the war. This parallels Finny's illusion that there is no war and that particular illusion's failure, as well as other illusions that fail throughout the work.
Leper's breakdown demonstrates the reality of war for the students as well as for readers. One needs only to see the suicide rates among today's soldiers serving in or having served in Iraq and Afghanistan to see the significance of Leper's change of heart toward the war for the Devon boys and the readers.