The museums in Chicago are huge. And we had gone to 2 that day.
That's about the same amount of walking as a day at WDW then - except without the cool, fun stuff at WDW
I'd have his blood counts checked at his next doctor's appt, just to rule out anemia, etc, and then maybe talk to your son about what it's like at WDW, the amount of walking, breaks, maybe set some ground rules and plans for him if he gets tired? And of course, rent a stroller if needed. If it helps you have a crank-free vacation, why not?