The knee joint wears out with age and it is the muscles around the joint that support the joint in the long run. If your father has had paralysis for the last few years in the left leg, then he has no joint supporting the left leg and as a habit, he must have been putting more weight on the right leg, so there is no good support on the right leg. Unfortunately, pain killers are a temporary relief measure and should be avoided if possible. Massaging oils also will not give the desired effect. He should get special padded knee caps made by some prosthetic professional which can provide support and use a walking stick to take the extra weight which the joint is not in a position to take.