A few ideas...

Document everything! If you have not been keeping a reading list, you may want to start now. It can be anything from a spreadsheet to a simple spiral-bound notebook, or a journal. Jot the date, title, author, and whether the book is fiction or non-fiction. If there is a reading level associated with the book, make note of that. Optionally jot any thoughts/reflections on how well the book was liked, and/or any new vocabulary words.

Make sure your child has a good dictionary to use at home when reading, and encourage looking up unfamiliar words.

Discuss what he is reading, ask questions, anticipate what might happen next, suggest thinking of alternative endings, prequels, sequels, etc.

To provide stretch outside the classroom, consider the crowd-sourced reading lists in this Gifted Issues Discussion Forum, in the Recommended Resources Forum. Children's magazine subscriptions may be another idea.

Have you seen the "Advocacy" thread? It is a brief roundup of crowd-sourced parent tips and experiences over time on this forum. http://giftedissues.davidsongifted....y_Advocacy_as_a_Non_Newt.html#Post183916

Be aware that "differentiation" only means that something is different, it does not ensure that the difference provides a better fit to your child's educational needs, readiness, or ability. Be especially alert to differentiation which consists of punitive grading. http://giftedissues.davidsongifted.org/BB/ubbthreads.php/topics/230047/Buzzwords.html#Post230047

Reading is wonderful, it is great to support and encourage it!