Don't bother going "up" the arch ... All the fun exhibits are under the Arch in the Lweis and Clark Museum.
Gee wiz....Do I gotta tell ya everything?
Tell him to get to the train museum, and the other sites mentioned. Kids will LOVE the hands-on exploratorium and exhibits too. Indian mounds are a nice-to-go-to-once site, but not worth more than two-hours walking around.