Hi!
Good post - good luck!
Form Closure questions:
What is the status of his pelvis - SIJ, pubic symphysis, L5/S1, etc. Does he have alignement issues / aysmmetry of his sacrum or innominates? Did he fall onto his coccyx / ILA of sacrum causing a shear of the sacrum?
Force Closure / Motor Control Questions:
How are his core stabilisers? Are his posterior pelvic floor stronger than anteriorly? is his coccygeous muscle overactive? Is he a "butt-gripper"? Does he have good lumbo-pelvic / thorax dissociation? Is there fascial / soft tissue imbalance? Also, what is the current status of his hamstrings? How are his load-transfer tests (ASLR +/- compression at ASIS, Pub Symph, PSIS, Ish Tub)?
It sounds like a nasty accident so I would be checking the above - only since you did not mention them...
The sacrotuberous ligament has attachments to biceps femoris - and sometimes the hamstrings don't even attach to the Ischial Tuberosity but go to the STL. It is continuous with the dorsal long ligament up to the iliolumbar ligament, whose main function is holding L5 onto the sacral base. Coccygeous is often overactive and can also be causing the pain. Just some thoughts...
Again, good luck!
Antony