You'll find tons of information on the forum about joint supplements and everyone has preferences for different brands. The most important thing with any form of degenerative joint disease is to stave off as much future damage as possible and the first course of action is a supplement with glucosamine and chondroitin. Most have other ingredients added as well, such as hyaluronic acid, and there are a lot of brands to choose from, so it's also important to find one that works for your dog. It can be a trial and error search. My dogs have always taken Acti-Flex K9 because I have seen results with it, but others will chime in with their preferred brand. Cas has elbow displaysia and he also gets sea mussel extract and Cetyl M, both of which I believe are helpful.
Best of luck to you both!
'If you find a way to enjoy the ride, it doesn't really matter where you end up'
5 GSDs at the gate: Katie, Iroc, Red, River, Jake