That's really hard - my DD1 was 13kg at 3yo and DD2 is now 2.5 and already weighs 14kg but they are both very different body types.

Funnily enough DD2 won't drink anything warm (it has to be cold) and prefers to eat cold things - she hates soup and things like that. If your DS isn't allowed to eat cold things can they be room temperature? It might be that he doesn't like warm things? Another thought - my kids like to prepare the food with me and then snack on it - maybe if you help get him involved with the food he might be more inclined to eat it.