I think this is unusual in such a young Boxer unless it is ill-bred (bad breeding can result in just about anything.) I would think it would have to be a rotating Patella or hip displaysia. My first Boxer had bad knees, but that didn't happen until he was about two and over-weight. I will tell you it is extremely important to keep a Boxer at a lean weight, don't let them jump up on furniture (I have footstools by my bed so they can walk up instead of jump) or into a car. Always have them put their front legs up and then assist them with their rear end/legs. This will help to keep them away from knee surgery. My Boxer had both of his rear ligaments replaced in his knees by the time he was three.