When you do test the joint, watch for a strong reaction of pain, and try to feel the joint, comparing it to the other good one if needed to see if anything feels off place.
More information about what you find on the physical exam will be helpful in determining the nature of his problem, but it sounds to me like he sprained/injured the joint, there could also be an infection, but just as a basic treatment, in addition to the rest, you can also soak the leg in Epsom salts every few days to help reduce some of the swelling. In addition, administering B vitamins, and omega 3's may also prove helpful.