i had tendinitis in my knees, i found out after a long hiatus from fitness. I tried to get back in shape and just running around killed my knees.
What I used, and what I recommend for your father to use on a consistent basis, is a dietary supplement made from dehydrated milk. I know it sounds weird but it actually worked well, its called Micosamine and it's specifically for promoting flexibility as well as joint function.
Here are some studies i dug up if you wanted to know about the thing you might put into your body, LOL.
Zenk JL, Helmer TR, Kuskowski MA. (2002) The effects of milk protein concentrate on the symptoms of osteoarthritis in adults: an exploratory, randomized, double-blind, placebo-controlled trial. Cur Ther Res 63(7):430-442
Hope this helps.