Reading Japanese

We read books to William as part of our bed time routine.  Since he's learned to read a bit, he sometimes wants to read the book himself.  We'd gotten some cool Shinkansen (Bullet Train) children's books that were written in Japanese, so we considered them purely picture books to look at the cool trains.  To our surprise, he picked these up and insists on reading these as well.  He's been playing with our Hirigana and Katakana (the two phonetic Japanese alphabets) flash cards and has learned the sounds that go with the characters.  But, it was still a surprise that he decided to try reading the books.  His pronunciation is probably poor, and we have a hard time helping him when he gets stuck, but he seems to cruise through the pages.  His reading comprehension, though, could improve a bit...