I'm not sure how long ago the MVA happened, or if they did a neck MRI at the time?
If not I would get the cervical spine MRI if his neck is hurting more.
If nothing else it will rule it/out if there is a obvious cervical spine issue. Could still be something small that won't show up..but things have been missed , it depends on the techs reading the images and if the dr actually look s at the images or just reads the report..

If nothing shows as a cause, then it most likely is soft tissue and you can proceed with PT and chiropractic or other therapies.
( oh I see whiplash was dx also?)
https://www.google.com/search?q=Whip...lash+treatment
If it is mostly soft issue pain around the neck and upper back, if you can find a treatment provider that has a low level laser (also called soft laser/cold laser) that might be very helpful.
IF stim is nice too ( just don't let then turn it up too high).

Whoever was at fault , their ins should be covering medical bills, I don't know the process of how to do that but their Ins carrier or the website should explain how to do it.