I Brought A German Shepherd Puppy Five Days Back When

I brought a German shepherd puppy five days back when I brought him he pooped hard n normal but now when he poops it is not hard but a little watery as the person who sold me the pup told me to feed him with cerelac until its first vaccine. The pup is fine he plays all good but I am worried if he is all right! N also I don't know other kind of foods should I give him.