One reason why The Wind in the Willows has an appeal to boys would be through its spirit of adventure.  Mole breaks free from his domesticated realm and enters a world where there is activity and energy.  Such ideas can be seen in the exposition of the work: “Something up above was calling him imperiously.”  He acts upon this, reflecting a spirit of activity and energy within him.  This appeals to boys in that it reveals a sense of wonderment regarding what might be.  For boys who embrace the idea of adventure, of seeing what can be from what is, this is one area where there is a specific appeal.  

I think that another realm in which there is appeal for boys would be in how friendship is associated with hijinks and a sense of experience.  Rat is extroverted, complementing Mole's sense of introversion.  Friendship is forged even though there is opposition.  Boys would find this appealing, as Rat takes Mole on adventures where mischief can present itself.  

Rat also possesses a self-reflective quality where he recognizes that his perception is fundamentally different from what he would want others to see and notice: “Do you suppose it is any pleasure for me . . . to hear animals saying . . . that I’m the chap that keeps company with jailbirds?”  I could see boys connecting with the idea that the way we perceive others might be fundamentally different from how we see ourselves.  For boys who are seen in "one way," there might be a desire to be seen in another, something that boys might address when they are in the company of friends.  This idea of humility in the face of self-understanding and awareness might be a quality that boys can appreciate.

In the end, all of these qualities can be understood by girls as well. However, there might be some intrinsic connection that boys can experience in Greene's work, as it addresses realities that boys often appreciate.