If it is indeed a luxating patella, it's a conformational problem, so it can't be treated, or cured by supplements or drugs. It can probably be managed by such. Off the top of my head, anything that's good for joint problems would be good for this, like glucosamine and chondroitin. The joint affected is the stifle and the loose kneecap moving around eventually causes arthritis. With very low grade conditions, it may take years and years for that to happen. With puppies it might correct itself, but in adult dogs it won't just disappear. It may or may not be a problem, depending on how active the dog is.
With that said, I didn't actually think to ask what leg is lame, and if it's a foreleg lameness, a patella won't be the cause
We adopted out a purebred APBT last October who had a mysterious on again/off again lameness. However, his lameness would last for a week or so and then disappear. And it switched legs. We x rayed and got nothing. The vet put him on Rimadyl, which is an NSAID, like say, ibuprofen or aspirin, only stronger. From what I last heard, the lameness has vanished. This dog was just over a year. So sometimes you can't discover a cause.
Your vet should be able to tell if the patella is loose by manually manipulating it. No x rays necessary for an initial diagnosis, so it shouldn't cost much. Now, surgery to correct it, that's another story. Very expensive, as all orthopedic surgery is.
As you may have noticed, I'm extremely long-winded :naughty: And I'm not a vet, but I hope this sheds some light on the subject.