I'm another food hider. Mum never listened to what I liked- just what she thought was "good" food. It got so bad that the teachers use to sit with me until I ate my lunch.
Could it be textures? some he can't deal with?

With my fussy eater, he gets a star if he finishes his meal (I give him small portions) he is super proud on days when he goes for 2nds.
Both my boys eat a 2nd dinner if they are up after 8pm, they get hungry- and 1 eats a huge dinner.
Perhaps its portion size related?

Its can be so tricky working these things out