156g for a 8 month old ball python is tiny. How often are you feeding him and what sort of heat and enclosure does he have?

To get a pied, you would need to get a pied(or at least a het pied), breed your pewter to it, hold back one of the babies to breed back to mom. If it's just a het pied(carries only one pied gene and looks normal), then the babies MIGHT carry the pied gene(and look normal) but there's no way to tell which babies DO carry it and which ones are just normals.