There are books full of stuff to make from Lego's.

http://www.amazon.com/LEGO-Ideas-Book-Daniel-Lipkowitz/dp/0756686067

http://www.amazon.com/LEGO-Play-Book-Ideas-Bricks/dp/1465414126

Klutz has a book

http://www.klutz.com/book/Lego-Crazy-Action-Contraptions

I might look for books of idea/projects. I found that science "kits" one can find are usually very simple, and what one is paying for is just the instructions and the convince of having all the parts already at hand. On the other hand books of idea's, that have different projects to make out of household stuff.

Your son might really like Maker Fair. http://makerfaire.com/ My son isn't that interested but I know a number of elementary boys who really LOVE it. They have a magazine. http://www.make-digital.com/make/vol38#pg1