My kid likes anything involving vehicles- airplanes, cars, air shows, races. They like zoos, aquariums, that sort of stuff. My wife will probably find a bunch of stuff by the weekend.
You may find that you don't need to go anywhere if he likes airplanes! One of the things I really like about MSN is the wide variety of traffic. You probably won't see any airliners on the East ramp on Saturday, they're usually there for sports charters for UW teams and there's not much going on during summer for college sports, but there's a chance. The actual airline flights are on the west side of the airport - Mostly RJs and A320 variants, but too far away to really see. There's also the 115th Fighter Wing F35s based on the South ramp that is much closer, but you probably won't see them unless they're out and about. However, since they're an ANG wing, they often do their flight exercises on Saturdays. However, all of the transient military traffic goes to the East (GA) Ramp, not the fighter wing. I've seen Harriers, Hornets, etc up close and personal there, and even one of the few C130s that has skis! There's a fair amount of bizjet traffic, and lots of training, transient and local GA. Kind of a little bit of everything there.
The other thing that jumped out at me is "zoos" - The
Henry Vilas Zoo is good, and free.
For races, there isn't anything right in Madison but there are events nearby at
Madison International Speedway on
Friday night with fireworks,
Jefferson Speedway on
Saturday, and
Angell Park Speedway on Sunday if you really want to make a weekend out of it! If you're into dirt track bike stuff there's
Aztalan for a future trip, but they don't have anything going on this weekend and it's closer to KRYV anyway. Another future trip idea for you is to go to Hartford KHXF and head to the
Wisconsin Automotive Museum, located inside the former factory of the
Kissel Motor Car Company.
Finally, there's also a good
Children's Museum that you can get into for free with your EAA membership, and if your kid needs to "get some wiggles out" after being in the plane, you could also stick 'em on a trampoline at Urban Air or Sky Zone but that is... NOT free.
Hope this helps, have fun! Wish I could be there.