Hi there
It can be because of degenerative changes in knee occurring because of age or any other reason like previous injury. I would like to suggest you to go for a X-ray of his right knee. And take it to an orthopedic doctor for further treatment or you can opt to go to a physiotherapist and he will take care of his knee joint by strengthening,stretching and mobilising the joint.
Next Steps
consult an orthopedic doctor or physiotherapist
Health Tips
apply ice pack t right knee for 15-20 minutes twice a day. take zerodol sp, shelcal 500, multivitamin and glucosamine tablets daily