Don't worry about it at all, It's normal for some dogs to cut back on their eating. Dogs are just like kids, they get fussy about eating. They know when their hungry or not.....don't force it on them, if he healthy other wise.

Some breeds of large dogs, Great Danes and Boxers you don't want them to "Grow like weeds" it's harder on their bones if their body gets bigger then what their growing bone can handle at their stage of growth.

Look up St. B on the net and I'm sure you can find out excatly how much food they need at each age and/or weight. Also check into what I said about their growth. It might make you feel better.