My almost five month old GSD male only weighs 28 lbs. Should I be worried? I feed him about 2 1/2 pounds a day, and he eats ravenously, is very active and happy, and stands 15 inches tall at the shoulder. I also work him monthly and am treating him weekly for tick prevention, since they're so bad this year. I don't care if he's on the small side, I just want to be sure he's healthy.