My partner just witness car lose control off car and rolled passed his car on the motorway, he now has back ache and headache why is this?

Find cause. The way the brain perceives pain often correlates with life experiences. If the headache is not the person's usual headache and is severe, and/or the back pain is not familiar and is severe, or if either persist for two weeks, a physician may be able to help find the underlying cause -- which went unnoticed until the shock of witnessing an accident.
Mimicking pain. Something traumatic like this may cause people to experience a conversion reaction. A conversion reaction or conversion disorder is characterized by the loss of a bodily function that is involuntary, but for which there is no biological explanation. If the condition then leads to nighmares and increase in symptoms upon seen similar events then most likely post traumautic stress disorder is possibl.