I also just thought of these two book/kits that my son loves. Your son would be able to play with these unsupervised, as they're just made of sturdy cardboard and plastic.

http://www.amazon.com/How-Cars-Work...r=8-1&keywords=how+cars+work+nick+arnold

http://www.amazon.com/How-Machines-...1&keywords=how+machines+work+nick+arnold


What is to give light must endure burning.