Whitney,
How exciting that you will be heading into Bigger with your son!

Usually when kiddos come out of the Emerging Reader's Set, they are ready to go right into Level 2 of the Book Packs. This is because the reading level range in the Level 2 Book Pack is actually 3.0 - 4.1 (meaning beginning third grade to beginning fourth grade, first month).
Here's a link to the package:
http://www.heartofdakota.com/drawn-into-optional2.php
If your child is really sailing through the Emerging Reader's, especially the end books (which are the highest reading level, then you could instead consider the Level 3 Book Pack. It's reading level is 3.5 - 5.1 (meaning mid-third grade level through beginning fifth grade, first month). The books in this set are also a bit longer with longer chapters and more text on the page. So, it is good to weigh how overwhelmed your child gets with more text and fewer pictures.

Often at this stage, how a child feels about the "look" of a book and the text on the page matters as much as whether or not he/she is actually able to read the words on the page.
Here's a link to the package:
http://www.heartofdakota.com/drawn-into-optional3.php
Blessings,
Carrie