Hello
Due to his surgery and the age factor, he might be feeling helpless and dependent on others for his needs. This fact might be bothering him and is making him feel low. You can try involving him in social interactions, make him talk to people, engage in activities he like to uplift his mood. If still you need some help from mental health professional , try consulting a a clinical psychologist or a psychiatrist for its management.
Health Tips
engage him in activities he like, social interactions, interaction with family memebers