14 Week old GSD feeding issue, maybe?

Question
I have a 14 week old GSD male.  He is weighs 31 lbs. now and is a great dog.  Listens well and is very social.  My only problem that I have is the fact that he is a very picky eater.  When we first got him a 10 weeks we were feeding him Iams smart puppy.  He ate fine for the first couple of days and then he got to the point where he would barely touch the food.  He'd rather go hungry.  I then switched him to Purina Pro Plan Selects puppy and he loved it.  It has been about 3 weeks and he is starting to be picky about eating it again instead of wolfing it down.  What may be the issue or is he possibly just not hungry?  He is fed a 3 times a day 1 1/3 cups per meal.

Answer
It is quite common for a Shepherd puppy to refuse to eat enough to completely hide its ribs.  4 cups a day sounds like a lot.  My 14 week old Lab is getting just over 2 cups a day of Pro Plan.  I just cut her back to 2 meals a day.  My bag of puppy chow is getting low.  As it runs out, I will mix in adult, until over a week it is all adult.  Too much, and to rich of a diet is the worst thing there is for a growing puppy's developing joints.