Ishmael needs to tell the truth: he’s 14 years old and suffers from Ishmael Leseur’s syndrome, which is incurable. His is the only known case of the syndrome. He tried, for a while, to believe it’s not a real thing, but now it’s unavoidable. To explain, Ishmael says his name is Ishmael Leseur—and yes, that’s the name of the syndrome. The name, though, is the syndrome.
Though it’s a bit convoluted, Ishmael is introducing the idea that he hates his name and blames it for all his problems. Coming up with this “syndrome” is a way for Ishmael to make sense of whatever problems he faces, and it’s a humorous way to convey these problems to the reader.
None of Ishmael’s family members—Dad, Mom, or his sister Prue—suffer from the syndrome. Prue, in fact, is “adorable” and almost a genius. So Ishmael’s name must somehow produce a virus that causes the afflicted person to do the most embarrassing things. And he knows who’s responsible: his parents. He might be able to forgive them if they hadn’t been laughing so hard when they gave Ishmael his name.
Part of becoming a parent to a brand-new infant is, of course, getting to name the new addition. So Ishmael suggests that he feels his parents abused their power as parents by saddling him with a name like Ishmael—and to make matters worse, acting as though his name is a joke.