Hi Amy,
A few of the books that my children, ages 3, 6, and 8 have enjoyed are:
Into the Sea by Brenda Guiberson
This is a non-fiction book about the life of a sea turtle from her hatching to her return 20 years later to the same beach to lay her own eggs. It’s told in a story, but with real facts about sea turtles
The Smithsonian Oceanic Collection
My kids love ALL of the Smithsonian non-fiction storybooks. The books are non-fiction with lots of “real” info about each creature, but they are told in story form in an exciting way. You can read a good review of them HERE. One of the nice things about the Smithsonian series is that most of the books are available with recordings that your son can listen to the sound effects and read along.
Octopus by Evelyn Shaw and Seahorse by Robert A. Morris
These two books are part of the wonderful “Science I CAN READ book” series published in the early 70’s. They are listed as K-3 and are for early readers. While they are out of print now, you may be able to find them at your library.
Happy reading! 🙂
~Anna