Personally for a snake that small I'd feed every 3-4 days until it gained some weight. I'd also double check your scale to see if it's reading right, and make sure the rodent is 10% to 15% of the weight of the snake. The less you feed and the smaller the rodent size the slower the growth rate. It would be helpful to post a photo of the snake, if he is still really skinny I'd feed more than one rodent per meal until he gets his girth back.
Some ball pythons are genetically disposed to get bigger than others but no one really calls them 'dwarf' or 'giant'. Some will get up to 5000 grams while others will never reach that size.