My fiancs back has been hurting so ive been paranoid and looking at it all the time. when he was sitting on our desk chair his legs were to the side and his torso was straight to the desk and the spine looked not centered. is this normal?

I have seen many people with spinal pain. I can't tell you how important an in-person examination is needed to help you. More details are needed; the nature of the pain physically and subjectively. Back pain can come from bones, joints, ligaments, discs and can be affected by the function, alignment and movement and of all of these.

He may have scoliosis that could explain the back ache and what you described as, "not centered". Visit this site for information and suggest that he see a doctor: https://www.mayoclinic.org/diseases-conditions/scoliosis/symptoms-causes/syc-20350716