Considering his age your father might have some degenerative changes in the knee which could be the cause of pain. Hence, a X ray would be required for accurate diagnosis.
Next Steps
Please visit the nearby orthopaedic or physiotherapist with the X-ray and other reports for detailed evaluation and essential treatment considering his symptoms.
Health Tips
If there is swelling along with pain, apply ice for 15 to 20 mins after every 3 to 4 hours.