Has he looked at our Summer Camp challenges? He doesn't have to submit them or anything, but there are some cool, fun but educational lessons in the Summer Camp Challenges... especially science or something.
If he gets any computer time or has his own, you might want to go to the Homeschool Corner, and the Afterschool Games pages. If you look at the last set of games, there are some really educational critical thinking games that are powerful learning tools... for example there are some where you "program" with little robots with functions, or others where you have to complete electical circuits. I can pull some of the best ones out and give you a list if it's something he could do or have access to... or something you might approve of for him!