He is a young dog so the first thing that comes to mind is a sprain or strain. This is especially likely if he is episodically active; the weekend warrior syndrome. If this persists, it could be a sign of something more serious. Larger dogs are prone to elbow dysplasia and panosteitis. Radiographs of the affected limb would be helpful to rule these out. Your veterinarian can also prescribe medication that will alleviate the pain.