Yeah I know how hard it is to please everyone. We were a family of six too growing up and mum had both fussy eaters and big eaters.

I think you need to look at the ultimate outcome you want. Do you want him to eat more in general, or eat more vegetables, or eat more at dinner time or just not hide food?

I would just tackle one of these outcomes. I personally think the hiding if food is a symptom of too much pressure to eat things so that might be solved by less pressure in general.

I would get some books about our bodies and the role of food in health and digestion. Maybe you could get a food pyramid type diagram and talk about food types and play a game where kids put a magnet on the type if food they are eating. In terms of health.if you focus on health and bodies and not quantity or specific foods then you might find he starts regulating it himself.

It shouldn't be a competition though. I would sell it as 'you are old enough to take some responsibility for your body and its health'. Just an idea.