Underweight GSD puppy?

i am have a GSD weighs about 1.50kgs or 3.3lbs . i got it from a breeder and it looked very small . when i went to the doc he checked his teeth and said its atleast 45days . i have put him on food supplement recommended by the vet and also feeding him eukanuba for small breed. i am seeing a bit of a improvement in him . now i am not feeling his ribs anymore . but still would love any other specific advice tht anyone of you have .