How can i fatten up my gsd pup?

i got a gsd pup about 2 weeks ago and hes SKINNY!!!......i can see all his ribs and i can also see his shoulder and hip bones clearly...apart fom that hes very active and he eats well...how can i make him bulkier??.....hes on bob martins vitamin supp. also..my vet said i should give him some blood tonic meant for humans but im quite sceptical about this...what should i do?????....i feed him dogmore dry food for puppies...about 125 grams a day..i split that over three meals...and i also clicker train him using hot dogs.....he averags 1 hod dog a day....wat am i doing wrong....HELP!!!