7month GSD has Giardia parasite infection?

Yes it's curable. He has an upset stomach and maybe doesn't feel well but the medication will really help. Feed him normally and let the medication do its job. My lab had it really bad as a puppy too and she recovered. However, she had a temperamental stomach for the rest of her life. If that happens you'll just have to be aware that her stomach can get upset if she eats things outside of her healthy dry dog food.