How did he land - on his head, shoulder, neck?

Have you considered an expert chiropractor- upper cervical perhaps?
A c1- c2 adjustment is very gentle if concerned about regular adjustments.
Skilled & experienced DC's can adjust very carefully & precisely.

If all Imaging & xrays are coming out as normal, this is the next step I would look into. those tests usually don't show the minute misalignment's that can be there. MD's generally don't study this area in depth...

And there is often still a bias against chiropractic..