Brother born spina bifida now 31 diagnosed with tethered spinal cord. Needs detethering how common/successful is it? Should he see dr in usa or sydney

Depends on symptoms. Neurosurgeons both in australia and the usa are quite capable of doing this. Practicality and logistics should dictate where it should be done. Success with detethering depends upon the symptoms that are present, and also the age at which symptoms present. Have a detailed discussion with the neurosurgeon, and perhaps get a second opinion. Choose one who does a fair number of these, at a sb center.