Hi there
He might be suffering from osteoarthritis and therapy can help him too but he need to follow do's and don'ts told by your physio. And need to do exercise daily at home as per prescribed by your physiotherapist. It's about strengthening his leg muscles so that leg can bear weight and pausing further damage to joints.
Next Steps
consult an orthopedic doctor
Health Tips
at home he can use ice packs if swelling is there or else can use hot water fermentation.
calcium, glucosamine and multivitamins tablets will be useful. avoid stairs climbing, long walks, and bending knee beyond 90 degree.