This brings back memories for me! For a few years, starting at age 2, our son was also enamored with all modes of WDW transportation. The monorail was his absolute favorite!
Unfortunately due to safety reasons, the WDW monorail no longer allows guests to sit in the front cab.
I have a tip though: When riding on the WDW railroad inside the Magic Kingdom, have your friends ask if their son can be the Guest Conductor. Read Disney Dad Marc's description of this magical moment
here.
Our son was randomly asked to be the Guest Conductor when he was younger and it's still one of my favorite Disney memories!
Other fun modes of transportation in WDW include:
Ferry boat, Seven Seas Lagoon boats, Sassagoula riverboat, bus, Tomorrowland Transit Authority, Tomorrowland Speedway, etc.
If the young car fan doesn't ride Test Track, he can sill enjoy the vehicle display. Just enter via the store at the attraction, then proceed into the GM car display area. Have the transportation enthusiast sit inside the vehicles! It's a thrill for young children!