Hello
I am doing my own trial/error with my own son now who has TS, but I do believe that getting regular ajustments from a chiropractor seem very helping in relaxing him and helping him focus better. He doesn't really get his back cracked, it's more of pressure points. The technique is called Torque Release Technique (TRT): TRT is called a tonal technique because it normalizes the tone of the nervous system.
From there website: Balancing the body structures through specific movements of the spine, or impulses to the central nervous system (called achiropractic adjustment) begins to release the natural healing power of the body.
Have you also tried taking a Magneisium supplement? Ive read it's very effective for some people with TS.