What Kind Of Food That Can Make You Gain Weight?

This question is not for me. But this is for my son. I still think that he is too skinny for a boy at his age. He is a picky eater. It is not so easy to serve food for him. Each time I always try to make new food. But it often end up with unsuccessful action. He will spit it out and say that he does not like that food. He only like few kind of food and he does not like to eat vegetables. Until what age that he will have this kind of eating problem?