Hi jmspbro:
I think adoptmama has made some great points - and I also suspect that if he's chosing from class library his options may be limited (he may have read everything in the box for him - but Y2/ Y3 children can want to read the same thing over and over if they enjoy it - we had a first encyclopaedia for 6 months in Y3).
The other problem we found is that first stage after becoming a free reader book choice is wide open but at the same time not clear. Your DS (like my DDs) may have loved the simplicity of short stories and clear plots and find chapter books (if he's moved on to that) a bit boring (no pictures or maybe not his thing as a genre). I know at our school we do not have a lot of non-fiction books and the boys are endlessly complaining about this. They want to know how things work, about dinosaurs, about the plantets, etc...
Chosing a book requires skills and this may be something you help your DS work on over the summer: What does he like in a book? Does he prefer illustrations? Does he have favourite authors? Does he prefer certain types of books: adventure, non-fiction, etc?
Some solutions:
Go along to your local library and see what summer reading schemes they have lined up. It may be possible for you to fire him up a bit about reading if he sees it as a competition with prizes. It may take a bit of juggling with your schedule - but this sounds a critical stage in his reading development so worth the effort.
Keep up to date on what books are out there (there's so many more now):
The Book Trust is great for this: www.booktrust.org.uk/books-and-reading/children/ - if you scroll down below the horizontally arranged photo icons - you'll see a list on the lift - under books for children there's book finder. Click this and then click 5-8 & you may also want to peek at 9 - 12. 8 is a really tricky age because 9-12 books may be too old and 5 - 8 books may be too young.
The guardian's how to build a classic children's library has a fantastic list of books to consider: www.guardian.co.uk/books/2000/mar/24/childrens.library - this is the link to the 8 - 11 recommendations with some advice on reading from age 8.
Tricks of the trade (developed over the years for DD1 (y5) and DD2 (y3)):
Go to book stores or newsagents (especially if you have a few minutes to kill) - let them browse & get excited about books and then discuss whether you've seen this book in the school library or local library. Often DDs are very persuaded by a cool cover illustration, which may not be the same at school/ library. But they will check out the book once they know about it.
Look into other forms of reading: Magazines and comics are fantastic. DD1 loves Pokemon and in an airport when we were delayed I cracked and bought her a comic novel Pokemon adventure. She was overjoyed - and has read it over and over again. But you pick up great vocabulary - she came home from her first day in Y4 and said "Mum, you won't believe it, I'm sitting with my arch nemesis at starter table" - Pokemon - all is forgiven.
If there's a book to the movie and they're old enough to see the movie - they're old enough to read the book (or have it read to them, which we've done for The Hobbit).
Finally - and because I think this is hugely important - does your DS see men reading (father, uncles, grandfathers, etc...) in his life. They have a role to play here too - and may also have good ideas about great books.
HTH