Love all 6lbs of your new dog! That is still a tiny dog in comparison to other breeds! Im sorry I wish I could be of more assistance. Noone can GUARANTEE the full grown size of a puppy for certain... not a GREAT breeder and not a BAD puppymill breeder. I have 2 boys, one is 5lbs and one is 8lbs. My 8lb boy is sooo much easier! I don't have to worry about his fragility! My 5lb boy is so tiny with little bird bones. Im afraid Im going to hurt him by accident hes so fragile. He has a harder time doing everything also where my 8lb bigger guy does everything for himself... I hope I made you feel a little better about the size if anything.
I would love to save EVERY puppy in the world from puppymills and bad breeders... but I can't. As hard as something is to do, the best thing to have done in that situation with all the questionable things the guy said to you is walk away without the puppy and find a reputable breeder that you trust. But you didnt, and you say you have a beautiful healthy happy baby boy... so at this point I would just love him to pieces and watch him grow up to be a beautiful healthy happy little man!!
Good Luck!!