That is one of the few curriculums that did not last at our house. It was just too much reading and too much information for my girls in 3rd and 4th grade at the time. It was not their attention span. I read to them for hours and was surprised by this response.
We have never been a big science family, however, and prefer reading fiction, biography and history. I used Considering God's Creation this past year. I am not recommending it, just that it worked for us because the lessons were shorter, usually 3 pages of reading with an activity.
If you like the book, save it for when he is a little older and try again. I think first grade is young for all that science information, but that is just my .01 worth! There are many animal books at the library with lots of pictures that you can read side by side and enjoy.