Personally I wouldn't worry if he didnt' eat all his dinner. I don't eat al my dinner sometimes so I don't expect my kids to. if they haven't eaten enough I tell them to put it in the fridge and if they are hungry later they can have it. Of course this is within certain boundaries. if they have taken one mouthful and just can't be bothered eating I tell them no eat at least 3 more forkfuls then you have had enough and then we get them to put it in the fridge

My 4 yr old will sometimes not want to eat because his brothers are finished or he wants to watch tv and latelt he is just to tired at meal times because he gets up so early and doesn't always nap so he asks us to feed him. occasionally we give in and feed him..