Wow! Sounds like a nasty accident!
I specialize in pelvic PT, and have seen many with coccyx pain. This typically begins after a fall on to the buttocks, and if he didn't sustain this in the accident he may previously have and this accident was the final straw. Be sure to check his pelvic symmetry (inominant rotations, sacral rotations, etc). I don't advise mobilizing for this, as you don't want to gain movement but instead want to gain symmetry. Therefore the best is repositioning techniques (muscle energy techniques). Also, a good stabilization program is imperative here that incorporates the pelvic floor, transversus abdominis, and eventually the gluts. Another forerunner to check is pelvic floor tension (if the patient is open to this, as it will have to be rectally) and especially hypertonicity of the obturator internus (which is an external exam). Release of this muscle (the obturator) is a must. I would recommend more of a "John Barnes" style for release versus the typical stroking massage. I follow this with high volt stim to the coccyx area (with four leads, bifurcation of two if necessary depending upon your unit) or NMES at 80 pps, 300-400 volts (for reduction of spasm--research backed for internal pelvic floor stim but I find it equally effective for obturator spasm) and moist heat (may use ice if more relief with it, but typically heat does the trick although everyone is different). A warm Sitz bath is advised for home management with use of donut pillow (inflatable) if the patient can purchase one.
My patients typically have instant relief (or symptom decrease) with the obturator release.
Good Luck and keep us all posted!
aletta