The Thorn Discovery that Michael rides is a 23"/20" frame, so the Captain/Pilot/Steersman needs to have a ride height of at least 34". I am 5'10" and I have a ride height of 36".
Hope he finds someone to ride with as he enjoys it so much. He tells us so, regularly on the Charlotte's Tandems Facebook site. Have a look and see.
Best wishes,
Alex