Nat Fantastic and the Brave Knights of Old by Giles Andreae, illustrated by Katharine McEwen

From chapters.ca:
Nat is no ordinary boy. He may look like it on the outside, but one almighty sneeze is enough to turn him into Nat Fantastic - miniature superhero! In this exciting follow-up toNat Fantastic, the small superhero is taking on the baddies - and winning! This riotously fun tale is perfect for all budding adventurers. ''Filled with high-action drama and wham-bam antics... perfect for boys who daydream about being a superhero.'' -Junior Magazine