Boxer's are medium sized dogs, they shouldn't be fed Large breed food. My pup is 5 mons and we feed Canidae which is an all life stage food. Puppy's really don't need "puppy food" they can eat adult food just as well. If they were out in the wild they wouldn't have a choice of puppy/adult food, they would eat what their mom's brought them! It's kinda a gimmic to get you to buy their food,which is just palin wrong. We don't supplement him with any vitamins, he gets all his stuff from his food and other foods we add to it. I am no expert, but the reson he is so big could be cuz you feed him large breed puppy food. That could cause him to have problems later on in life like hip dsyplasia, and such because he has grown too big, too fast. How old is he? You can try doing a search on here to find more info on this subject, it's one of the blue buttons in the top right corner! By the way, where in Abingdon are you, I live about 5 mins from there?!:D