My most successful middle school science fair had me experimenting with different liquids to see which would exhibit the most capillary action. I made it a little more scientific by using different concentrations of sugar water, but he could also use juice or soda or things like that.<p>Seriously, though, the kid should be expected to come up with ideas on his own. Go through his science book with him and see what he's learned about that's most interesting to him.