His Back got Stiff due to Long Hours of Lying on Bed. It's common after surgeries. As he can't do much Movements, some simple Back Exercises can be performed to Relieve Stress on his spine.. back Stretching can be done and Movements like rolling on bed& Bridging Exercise will help him.
Next Steps
its better to consult to Physiotherapist for these Exercise, for Better Results and Right Treatment.
Health Tips
don't do excessive Exercises, These are done under the limit of pain.