Yamin, with one of my DS I am so with you, like your dc he's naturally slim, active and petite - his brother is a natural enormous giant
However, he just can't eat "3 square meals a day" as his appetite is too small, he's one of life's natural grazers. I just make sure that in his lunchbox and at home he has access to a range of healthy foods. Oh, and he's also a non-meat eater (the rest if us aren't) so ensuring he has a good supply of alternative sources of protein is an ongoing project!
Pasta with pesto, spinach and feta
Banana muffins with bran, honey and seeds
Home made mackerel pate
Lots of fish - Nigel Slater had a great recipe for marinated salmon with sesame seed crust.
All have lots of brain boosting elements.
I truly believe what you eat affects how you feel and how you perform.