Most of the free older books I can think of require a higher reading level.
You can look through:
Project Gutenberg on the
Children's bookshelf
Young Reader section on feedbooks .
There is a
children genre section on MR .
Starfall is an online site that has fun reading stories at your son's reading level.
I haven't used the below sites, but you could check them out..
A website of older books that can also be printed out.
http://www.childrensbooksonline.org/
This is a thread that was posted on MR earlier this month...
Hi, free animated picture books for kids and a request for feedback
Plus a storybooks for Children online
http://www.magickeys.com/books/
Hopefully, a member who is a parent (with younger kids learning to read) will jump in with some better suggestions for free txt, pdf, or lrf format books.
