He has luxating patellas. It's very common in Yorkies and other toy breed dogs. Don't panic. It can wait until Monday to see your vet. If he's got arthritis, he's had them for awhile. Your vet will grade them from 1-4 according to severity. 3 and 4 are the worst and will require surgery.
Here is some information for you:
Luxating Patella: A Knee Problem in Dogs