Leper is different from most of the other boys because Leper could best be described as a naturalist and/or idealist.  He is a peaceful and quiet kid that enjoys spending time in nature and collecting snails.  Nothing really upsets him, either.  He's not easily angered, and he isn't likely to react on sheer instinct to a changing situation.  He's a pensive and methodical thinker.  He's also a bit of an old soul.  I wouldn't say that he's against change or technology, but there are times when Leper lets it be known that he thinks new inventions are ruining his favorite activities.  It's why he speaks up against downhill skiing.  

They're ruining skiing in this country, rope tows and chair lifts and all that stuff. You get carted up, and then you whizz down. You never get to really look at the trees, at a tree. I just like to go along and see what I'm passing and enjoy myself.

Oddly enough, it's the war propaganda film about skiing soldiers that probably does the most to convince Leper that enlisting might be a good idea.  

Nothing tainted these white warriors of winter as they swooped down their spotless mountainsides, and this cool, clean response to war glided straight into Leper’s Vermont heart. . . 

“I’m going to enlist in these ski troops,” he went on mildly, so unemphatically that my mind went back to half-listening. Threats to enlist that winter were always declaimed like Blinker’s, with a grinding of back teeth and a flashing of eyes; I had already heard plenty of them. 

Gene doesn't believe that Leper will actually do it.  He just cannot envision a kid that collects snails and hunts for beaver dams enlisting in the war.  Leper is a kid that goes hiking and camping in order to more or less commune with nature.  He's happy when he is in a quiet and solitary location.  War and combat are antithetical to what Leper enjoys doing and being around.  Gene has no problem envisioning boys like Phineas joining the war.  He loves competition, but Leper is not like that.  He even refuses to jump off of the tree branch.  Gene is so floored by the fact that Leper joins the war that Gene actually feels that the war is even less real now because of the unreality of Leper joining it. 

So I ceased to have any real sense of it.

This was not shaken even by the enlistment of Leper Lepellier. In fact that made the war seem more unreal than ever. No real war could draw Leper voluntarily away from his snails and beaver dams. His enlistment seemed just another of Leper’s vagaries, such as the time he slept on top of Mount Katahdin in Maine where each morning the sun first strikes United States territory. On that morning, satisfying one of his urges to participate in nature, Leper Lepellier was the first thing the rising sun struck in the United States.

Leper...