My vet diagnosed Jezebel with a grade 4 in both knees, which shocked me because I had NEVER seen her have any problems with it. No limping or carrying the leg, nothing. I thought she was full of it but held my tongue--a dog with grade 4 can hardly walk.
A couple of months after that, she injured the knee (torn ligament) and I took her to an orthopedic specialist/board certified surgeon. The expert said that both knees were a grade 2. Big difference! She did require surgery due to the torn ligament, and while that was being fixed he stabilized the patella as well. But the surgery would not have been necessary for just the grade 2 LP alone.
(Disclaimer I am not a vet!) If you read a description of the stages in this link, I wouldn't think your dog is a stage 3/4:
Knee Problems In Your Dog - Patellar Luxation - Luxating Patella
Waiting, however, as the vet suggested is the correct advice. Just try and limit jumping up or down. Lots of dogs go through a long life with stage 2 and never need surgery. If you want a second opinion, you can always ask for a referral to a board certified orthopedist/specialist and get an evaluation.