dev. of 13 week old shepherd

Question
just got a 13 week old male GSD. Seems healthy, 2 trips to vet by breeder. Very small. Eats well, all systems seem function fine. Very alert, not anxious. Seems intelligent. With proper diet, can this "runt" catch up. Appears to be more size of 9-10 week old.
Took a chance as he stole my heart.

Answer
I am afraid I can't promise he will reach average size.  Even out of the same litter, adults vary in size.  There is little you can do with his diet to affect his adult size.  Stick to a dog food and very little else.  Supplements will not increase his size, but can interfere with the proper development of his joints.  The worst thing is too much calcium.  Most puppy chows by analysis already are close to the 1.5% maximum.