Dr. Greene's Answer
Kids will often find a food they enjoy and stick with it almost exclusively for a while. Apples and other fruits and vegetables are great choices.
The biggest problem from apples might be loose stools. There are times that a child’s cravings do seem related to nutrients they need, but more often it seems to be that the sensation of the foods is comforting, pleasing and familiar.
If his stools are fine and he gets a multivitamin (just in case he is missing something) and he gets some protein every day, I don’t see a reason to worry.