Try adding some toppings to it.
Like wet food, treats, etc.
Sometimes even fish oil can add enough for a picky dog to eat it.
My dog gets two meals a day. 1 cup in the am with wet food and 2 cups in the pm with wet food. That is all i can get into her usually. Sometimes i can add another cup in the pm. But the AM she is super anxious because she knows im leaving and needs me to sit right next to her and put the bowl on my bed in her face or she will not eat, and 1 cup is the most i can get in her in the AM! And if she doesn't eat and exercises at all that day, she usually vomits bile. So she has to eat, so i do whatever it takes
Even if the ratio from dry and wet is way off than normal.
Zelda is a healthy weight i think, especially with have bad hips she is on the lean side, which is what i plan on keeping her. She was eating about 5 cups a day before, but her growth has slowed down, and so maybe she just needs less.
You can also buy a food with a high protein content and fat than the one you are now, so that you can feed less and still get the nutrients he needs.
Perhaps your kiddo is done with most of the growing and needs less anyways, and his body is letting him know so he is not eating as much.