Knees can be difficult to help as they take a lot of our weight. To be honest, the cod liver oil might not be strong enough to help much, especially if you are talking capsules rather than liquid.
Since you mention both knees, there is a chance the cause could be Rheumatoid Arthritis, since this typically affects the same joint on both sides of the body. Osteoarthritis (wear and tear) and injury tend to be one-sided.
Unfortunately, we can't definitively say which is here though, although his doctor can test for Rheumatoid Arthritis.
No matter. If your father in law is willing to try the following, he may well help either cause: Keep a food diary for a week and see if the pain worsens in the 12 hours after eating the following foods - tomato, potato, aubergine, bell peppers, wheat, dairy or citrus.
At 73, most people would struggle to break down certain substances in these foods, which can cause a reaction.
Try to get your father in law to eat oily fish as often as he can. Quite often the beneficial oils don't upset the tummy when supplied by proper food.
Ensure constipation isn't an issue, since we know there is a link between gut bacteria and some forms of arthritis. Try Linoforce if necessary in the short term and lots of lovely winter weather stews and casseroles, full of root veg, beans and pulses for the fibre. These foods both help bowel regularity in a gentle way and improve the bacterial environment in the gut.
Your father in law can safely apply Atrogel® and take Atrosan® without fear of a clash with any medicines he may be taking.