Uni was diagnosed with grade 3 last summer. I have had her on Cosequin ever since, and she has not lifted her leg once. Now giving supplements does not make the LP go away. It just helps lubricate the joints. If you dog is developing lameness then you need to have him seen by a specialist. However, like Patti said, many dogs have this disorder and life long normal lives without having surgery.
How old is your little one and when was he diagnosed? It's also best to keep weight under control as too much weight on their frame is harder on the knees.
Take a look at this site, maybe will help to explain a bit better for you:
OFA: Patellar Luxation