Your symptoms suggest intrarticular soft issue pathology.
Try:
1. Regular knee quadriceps exercises ( static and knee bending).
2. Ice pack application will help
3.Medications like Cartigen DN and Enzomac forte will help.
Next Steps
At 76, surgery is not needed as the lifestyle itself becomes sedentary.
Try strengthening knee muscles instead so that they take over ACL function.
Health Tips
Treat him for osteoporosis and osteoarthritis.