SPINAL SHUDDERS - DORSAL RHIZOTOMY

My son has spastic diplegia and underwent a Dorsal Rhizotomy at age 3.  He is now 16 years old, walking without assistance and doing well.  Every so often he would experience a "shudder" down his spine - a result, we figured, was due to his maturing, changing body and the compensation his nervous system had to undergo after surgery.  Recently, however, these shudders have become more frequent and more physically notable.  He noted that the sensations pick up dramatically from his neck right down to the incision site (lower mid-section of his spine) and that if he cannot shake off the shudder (by physically lifting his shoulders and shaking his upper body himself) the tingling will go all the way down to his toes and last approximately 1 min. or less.  During this time, he said he feels unable to move.   If pressure is applied to his back or his back is rubbed with some pressure, this seems to ward off the shudder.  We are concerned with the frequency of this sensation and the overall effects and would like to have this checked by a doctor.  What type of specialist would be appropriate - Back/Spine or Neurology?  Do these "shudders" represent a red flag to problems that need immediate attention?   Many thanks for you kind review & advice.