I'd be giving him four meals a day until he is 4-6 months, then give him 3 meals, and then start him one two meals when he is a year. I'm not sure how many cups to feed though...

Because he has GSD in him, I'd skip the puppy food and go right on to adult food, or atleast give him half puppy and half adult. GSDs are prone to ALOT of bone/joint problems, and puppy food is filled with excess nutrients that can make the bones grow too fast. They should grow at a slow and steady pace to ensure that they do it properly.