You should consult an orthopaedic surgeon first who will advise some x-rays and tests and then depending on the disease and the stage of disease further treatment should be done.If the disease is advanced i.e. stage 3or 4 the supplements might not work and some surgery might be needed.

There are plenty of supplements in the pharmacy which will help the joints to some extent but don't take them blindly.get assessment by orthopaedician and x-rays before following any treatment of your choice à

Considering his age your dad seems to be mostly suffering from osteoarthritis knee, depending on the stage of osteoarthritis treatment is based, treatment vary from medication, exercise and viscosupplementation for mild to moderate osteoarthritis to Knee replacement in severe deformed knee osteoarthritis, kindly consult orthopedician and get a xray done to know his stage of osteoarthritis.

Considering his age and the brief history, my first guess would be osteoarthritis. Whether or not his case can be managed non-operatively depends upon the stage of the disease. I'd recommend you to consult an orthopaedic surgeon for clinical examination and x rays in order to ascertain whether this can be managed conservatively.

Yes, there are a lot of supplements and non-surgical interventions which can only alleviate the pain and suffering of the patient.