Crazy idea here, but what about using a chair/ short step stool instead of the 10' ladder and going for comical with him clinging for dear life 1-2' off the ground? You'd still need a safe place to rig a rope and some competent adult supervision, but that would be a lot safer (especially as you could even get the track coaches involved and use mats for rehearsals), and wouldn't require a flying rig.
There's still whatever that rope he is hanging on is hanging from that you'd have to deal with safely.