Science for the very young?
sga: You could have a lot of fun with an inexpensive microscope (look at a number of different materials, bugs, etc..) or even a set of magnifying glasses.  Get your hands on some polarizers, play with the affect of one and your ability to look into bodies of water (pool, lake, etc) show him that if you cross the polarizers you can't see through.  Couple the polarizers to the microscope and do some polarization microscopy.  You could also play with prisms and look at the dispersion of light.  Lots of good optics stuff out there.  I would highly recommend staying away from lasers until he's older.You might also consider doing some crystallization experiments (google "crystal projects for kids").