I recommend that you start your puppy immediately on the Bone Healing, Bone Maturity Mix.
Once the cast is off, if you can source comfrey leaves or roots, mash them in a blender mix with linseed oil and apply them as a poultice to the broken leg wrapped in cloth. Once it falls off repeat the process. Comfrey stimulates the soft and hard tissues to replicate themselves to normal, strong, healthy tissue (not to be used on an infected wound).
Diet is really important. I recommend that you place your dog on a diet of Raw Meat and Bones i.e. beef/mutton bones with meat on them or raw chicken necks and wings, mince; definitely no processed food and snacks. You can also add some freshly cooked vegetables and rice.
If your dog is not already on this type of diet, then I suggest that you introduce it slowly.
The following articles discuss dogs' diets and why it is important to keep it natural and simple:
Herbs, Common Sense and The Digestive System of a dog