This is difficult to say without more information. It may be a good idea to discuss this with your husband. A follow-up visit with the doctor might be the next step.
Sometimes there are psychologic reasons for this kind of behavior. When people have a back injury, it's not uncommon to avoid moving in any way that may cause pain. This is called fear avoidance behavior. A physical therapist can work with the person to overcome this mental block.
Emotional stress and depression can add to the problem. Studies show physical activity and exercise can help. Ask the doctor for approval and consider becoming an exercise partner. This may help encourage your husband.