The best advice a doctor ever gave me when we were worried about the amount of food our youngster was eating was this:
"No healthy child who has access to food will ever go hungry."
Sounds simple , but it helped us in 2 very important ways. One , it stopped us worrying about how much or how little he ate. Somedays he would eat very little , somedays he wouldn;t stop. And he would ask for food if he wanted it.
Secomdly, it gave us the opportunity to control his diet. At first we were mollycoddling him and he was becoming fussy and picky. So we would provide good, healthy food, taking into account the foodgroups etc.
After an initial period of moaning fussing and the odd night of going to bed with no dinner, he began to eat what we put in front ofhim. And if he only ate half, well we wouldn't make a huge fuss.
He eats well and the plus side is that I eat better now,becuase in forcing myself to provide good healthy food for my child, I found my own diet improved.