You have a very real concern. Your father will be filling out forms at the doctor's office. Then there will be more forms at the hospital or center where the surgery will be done. You may want to go with him. Make sure he understands what he's reading and signing.
Sometimes the doctor has a video patients can watch. For example the American Academy of Orthopaedic Surgeons puts out a video called Arthroscopic Knee Surgery: Return to Action. Watching a video helps increase patients' understanding of surgery and what's expected.
It may be helpful if you can let the doctor and therapists who will be working with your father know of your concerns. Extra effort can be made to make sure verbal and written instructions are given at his level of understanding.