My son has a good arm, but he just cannot catch.  Duke, what can I do?

Well, don't worry about it.  It is like this with most kids at an early age.  The way that i find to teach young players to catch is to have them place their thumbs to the sky and about shoulder height.  When you toss the wiffle ball toss it from a close distance so that you have control of your thow and get some air under it so they can see the ball and they will have time to react.  Try to have them say "Catch", when the ball hits his or her hands, and this will reinforce them to watch the ball all the way to their hands.  Make playing ball fun for the players so if they are really young just use 2 rolled up socks so that the object is light and won't hurt if they get hit, so that you take the fear of getting hurt out of the equation.  Make sure that when they catch the ball you clap and get excited for them becasue is what will make them try even harder to catch more.  Play and have fun.