Why is it surprising that Leper is the first Devon boy to enlist, and how is he different from the other boys who talk of enlisting?

Leper is the least likely to enlist for several reasons.  First, when the other Devon boys discuss the war, Leper shows no interest.  Instead, he is fascinated by his snail farm and other elements of nature.  Secondly, Leper shows little propensity for athletic or physical activities.  He has no desire to jump from the tree, and when Finny announces that Leper has finally decided to jump, Gene is shocked because it seems out of character for Leper.  As Gene and Finny excel in Blitzball, Leper fails, and it is only because of Finny's generous nature in interpreting Leper's lack of skill as a new play, that Leper continues to play.

Leper is different from the other Devon boys because most of them are interested in true competition.  They see the war almost as another venue for competiting with one another.  Leper is an Idealist.  He wants to commune with nature and has a tendency to romanticize real life--hence, his decision to enlist after he watches a propaganda film about the ski troops.  Leper, like Gene, is also an outsider, but he doesn't have Gene's physical abilities to create a true "in" for himself.  Part of his decision to enlist first is to fit in.