Hey! I heard of this torture method they used back in WW2 where they bend your arms behind you then hang you against a wall like that. It sounds similar to the hanging by the thumbs though, so IDk if you'll use it.
You could also do kneeling on dry rice grains, or the pear of anguish (A pear-shaped instrument thing was inserted into the anus/vagina and slowly expanded by the torturer by turning the screw at the other end.)
OUH what about pieces of bamboo stuck under his fingernails/toe nails? Or impail his fingers with bamboo...
Anywyas, amazing story! Hopefully these help. :3