Hi Robert-
While there might not be specific nutritional requirements for an 18 month old GSD, many choose to use a food geared toward large breed adults because they tend to be formulated with lower fat and often times larger pieces of kibble. I personally do use large breed adult formulas for my pitbull and lab for those reasons. When my lab eats a small kibble it falls out of his mouth and makes a mess.
18 months is the correct age to transition him to an adult food, so your vet is correct. Dr. Tim’s, Victor, Fromm or NutriSource would be my suggestion.
