Joint health

Hi! I have an EM x GSD about 140lbs and approaching 2 years old. He is on the lankier side and I have noticed that his back legs are the slightest bit shaky as if he doesn’t have a lot of strength in the back end. I’m wondering if any of your veterinarians have recommended any exercises or supplements for your young EMs. I do have a few bottles of Cosequin max that I grabbed on clearance. But I know most vets don’t recommend supplements until dogs show signs of aging. I also don’t want to give him any supplements that will cause an imbalance. Thanks in advance!