He needs to be seen by an orthopedic vet and have his knees evaluated. If they are Grade 1-2 you will probably just need to keep him from jumping on and off furniture and add a good glucosamine supplement. If they are a Grade 3-4, he will need surgery as the joint will only deteriorate more and arthritis will set in. It will become very painful and eventually, inoperable.
Here is some information for you:
LUXATING PATELLA (KNEE CAP)