Hello, we are looking to purchase a German Shepherd puppy who will grow to full size and be a personal protection dog. We have found a young boy but at 10 weeks he is only 4.8 pounds. If I'm not mistaken this is rather under-weight? Will he catch up or will he never grow to "full size"? The seller does not appear to be informing us that the puppy was the runt of the litter and is attempting to charge full price. How much would a puppy like this be expected to sell for usually?