rescued a German shepherd

Question
I just rescued a 2-3 year old German shepherd. The owners who previously had him hardly fed him and when they did it was just scraps of food. He was living outside tied to a tree for his whole life. He is very much under weight, he is just under 52lbs. You can see his ribs and his hip bones very easily!! I just wanted to know the best way to get him to gain weight. I know that i should feed him a few meals a day, but how much should i be feeding him? the recommended for his supposed to be weight or more so he gains more weight? right now i am feeding him eukanuba for German shepherds, but normally and i will be (once he is done with this bag of food) feeding him blue buffalo, a very high quality food. Is there anything more i should be doing for such a under weight dog? I have brought him to the vet got his shots and dewormed. He was diagnosed with heartworms, but we have to save up the money for his treatment. I just want him to be healthy, so whatever you can tell me please do so.
Thank you!

Answer
I would pile on the Eukanuba as long as he will eat enough of it to gain weight.  If not, you might try the puppy version of it.  If he still won't gain weight, try a small breed puppy chow.  As far as I know, nothing else is as high calorie.  I would hope 5 cups would be enough, but I had to feed one Lab 7 cups of Pro Plan to put weight on her. Being a Lab, she ate it and looked for more.  She was a high energy, field Lab, and came to me under weight at 7 months.  Another Lab, I had to feed 6 cups of Pro Plan to keep his ribs out of sight.  Shepherds sometimes are picky eaters, refusing to eat enough to completely hide their ribs.  If you get him up to only a little on the thin side, and he refuses to eat enough to gain any more weight, let well enough alone.  

Thank you for giving him a good home.