I agree with the consensus that your pup has ricketts.
Ricketts essentially develops from 2 basic problems:
poor nutrition and
lack of sunlight.
If you are feeding one of the cheap, kibbled feeds (Pedigree, Puppy Chow, etc.), then already you are feeding something that is essentially grain-based and devoid of the proper nutrition that a developing
carnivore needs to achieve optimal health. Corns, wheats, glutens, and beet pulps (something all cheap kibbles are made of) are simply inadequate nutrition for a dog or puppy.
Second, seeing as it's wintertime, many owners keep their puppies indoors and/or in their garages or basements, which means the developing puppies aren't getting any sunlight. Sunlight is essential for the body producing and utilizing Vitamin D, which in turn is essential for the body's proper assimilation of calcium.
Therefore by depriving a pup of sunlight, and by feeding them cheap kibbled feeds, is how basically every case of ricketts develops ... when an owner suddenly finds themselves with a malformed puppy, right about at the age your pup is.
Fortunately, ricketts (if caught in time) can be completely reversed, and (you guessed it) the best way to reverse the process of ricketts is to (1) provide your pups with some time in the sun every day, (2) to feed a better-quality feed, and (3) to supplement with Vitamin D and calcium. As has been suggested, the Vitamin D can be supplemented with a dash of cod liver oil, and the calcium can be supplemented with a nice glop of yogurt added to the feed.
And if you're feeding kibble, what would really do your pup wonders would be to throw kibble away forever, and instead switch him to raw, whole chicken quarters, bones and all, whole soft-boiled eggs, shell and all, and to never feed kibble again. But that is a whole other subject
Best of luck.