i would get a different set for new readers than the ORT - even if he likes the stories and characters, they are very samey, and after a while, he'll be regugitating rather than reasing.
Even my Oxford Rep says that you're better to read other stuff at home.
so, what about the Bananas (in levels of difficulty it goes Green, Blue, Red then Yellow), or Ready Steady Read, or Ladybird Read-it-Yourself (levels again), or Usborne Very first reading - they're great, and quite new - you start off where the adult reads most of the text, and the child reads some of it (based on Phonics levels), and as the series goes on, the child reads more and more of the text until they read the whole book themselves. (they can also go back to the earlier boosk and read all of that text too)
there is a box set for those: info