Was his neck checked out very thoroughly at time of injury?
MRI or Xrays?
And was there any PT assessment/evaluation for neck strains /spasms if no bone or disk issues showed on imaging?
They often say whiplash might not appear right away, so could be some of that happening now..
and If you haven't seen posts about upper cervical chiropractic,, that might be worth looking into.. a site search will find that info for you.. or google...

Take care of the flu symptoms for now, as it is very bad this year..