I adopted an abused purebreed doberman puppy he only weighs 16 pounds and is almost 5 months. He is healthy otherwise but I don't know if he will be a large dobie.
He is absolutely way too small / underweight.. My black FEMALE puppy at 10 weeks of age weighed 21 lbs, her sister was nearly 18 lbs.
You should take him to the vet.. Get him on good food.. EXCELLENT food, no crap at all..