What Causes Severe Pain After A Whiplash Injury?

my husband has whiplash from being rear ended 2 1/2 weeks ago. he had two discs fused 3 months ago. x-ray and mri show everything is ok there but he is still in alot of pain and his Dr does not want to give him anymore pain medicine since the imaging was ok. Almost like they do not beleiv he is in pain. Should he seek a secong opinion?