Over-eating

At what age can a child begin "over-eating". I've heard when a baby is hungry, fed him. But when does that stop lol. My child eats an ENORMOUS amount of food. Adult portions for meals. He's still so tiny-- only 20% percentile for weight and 66% for height. But I sometimes worry about forming bad eating habits. When he eats a ton it's very balanced and includes proteins, grains, fruits, dairy and some fats. But like, tonight he ate like 6 fish sticks and probably a cup of broccoli tots (broccoli chunks and cheddar mixed together and formed into a tot shape) and a cup of peaches. And was still asking for more. Am I silly for being concerned or is he still young enough (17m) that if he's asking for food to just give it to him?