Positioning depends on where the retinal tear is located. He probably has a gas bubble in the operated eye. If the retinal tear is on the upper left retina of the right eye, then lying on the right side will allow the bubble to be in close contact with the tear to support it in place while the tear heals. Do not hesitate to call the doctor for clarification if you are not sure about the instructions.
Call the Doctor and re-confirm that he wants him on his RIGHT side this time. Sometimes docs and nurses make mistakes. Don't take a chance on this, talk it over with him until you are perfectly clear and comfortable you understand which side and exactly why it would have changed this time.