Hallucinations occur when someone is awake.
Some medications, like atypical SZ drugs, cause vivid dreaming.
You or your husband should check with his pharmacist to see if the medication he is on causes vivid dreaming.
Some people do have the same dream over and over at certain points in their lives. Perhaps your husband has some subconscious anxiety about taking the medication or about the effects on his body and his life as a result of the surgery.