Hi, Jean,
If your pup is rhe son of a GSD cross the chances are that he will be quite big.
At this stage I would 'feed to appetite' even if it means wasting a litlle food left over after each meal.
I wouled also suggest half and half dry food and moist. Feed one kind in the morning and the other at night.
Make sure that you de-worm him regularly too, until he is six monthsd old.
W