The first thing that comes to my mind is Pinewood Derby cars. (This is kinda a brain storming idea so I didn't think it all the way through.) Maybe bring a piece of track and some old cars. In 8 years as CM, I have collected 5 or 6 unclaimed cars. You could also bring derby rockets or raingutter reagatta boats. How about a "Knot Board", Den Doodle, patch collection, hiking stick w/ medallions on it and a belt with sports and academic loops on it. (Ooooh, shiney things!!)
A snack you could provide is a no-cook s'more. Get the little paper mini cupcake holders, they're about the size of a quarter. You put a small square of graham cracker on the bottom, then a small dollop (sp?) of marshmellow cream and top it off with a piece of chocolate. You may want to check with your school first as many schools do not allow such homemade treats to be brought on campus. (I would ask permission in this case as I would like to be invited back again.)
Ask your son what gets him excited about being a Cub Scout.
You should cherish this invitation. Most of the schools in my area do not allow any off campus groups to recruit on campus. Gook Luck and have fun!!!