If the puppy can't stand you are in deep trouble.
If you don't correct it NOW the puppy may die. The flattening of the chest squashes its vital organs. I have had one pup that had this and she is now a normal young dog. I searched on the internet under "swimmer puppy" and found a wonderful site that showed how to make a jacket for the pup so as to train the front legs to grow in a downward direction. the site is at
www.swimmerpuppy.com My pup was only 4 days old when I felt the difference and went into a panic. Do something NOW for the poor thing before it's too late. 12 weeks is a long time to let it go. Don't leave it another day or you will loose it.