We found a cool catapult design that he might like, when DS7 was doing them for DI. Here is the site:
StormTheCastle

It's a little one made of popsicle sticks, but I got him some sticks about the size of yardsticks to make a bigger version. He never managed to finish it, and his friend took the sticks to finish it himself, but it would have really flung well! I'd like to see one in action, so I hope the friend makes it. smile It's based on the strength of triangles/pyramids.