I don't even hesitate to say.... whatever the lad is interested in at the moment.
You know...those spontaneous Mom questions "mom, why doesn't God make it so everything can change like a grouper can?' Wouldn't that be handy mom?"
I can't remember...ah.. now I remember. We were reading one of the Riley adventures books.
This process is called sequential hermaphroditism. Which I told the lad but he promptly forgot. :) But he found it interesting indeed...even where it is mostly aquatic species that have this ability. YaleNews was helpful in explaining why more animals don't do this.
Anyways, I find doing this kind of learning the easiest. It is spontaneous. it is easily remembered. AND it is good learning.
By the by...if you want books that can be good conversation starters among animal interested children...these Adventures of Riley books are excellent!