There's a really wonderful man out there who has published a series of books for parents and their children to use at different ages. I'm using them now with my younger grandchildren, one of whom is going into 3rd grade, the other into 7th.
For your son, I'd use both the fourth grader and fifth grader books -- the fourth grader one during the summer to patch any holes left from fourth grade; the fifth grader one as you go through the fifth grade year. You'll find all kinds of things in there that may or may not be on your particular school's curriculum, but if you work with your son on these things as enrichment, you'll all be much happier as fifth grade goes on.
On Amazon.com, these are available from re-sellers -- which is terrific because they cost far less than when they first came out!
Right now, I'm using the ones for second and third graders with my younger grandchild, and the ones for sixth and seventh graders for the older one.
Sites about Hirsch: http://www.google.com/search?q=e.+d.+hirsch+jr.&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a