How To Make Him Gain Weight

I really have no more idea anymore how to make my son gain his weight. When I saw him this morning, I really have to blame my self that I could not make him gain more weight. I cook many kind of things, I try to give him the best food that he can eat, but he really choose what he will eat. If he does not want to eat the food, he will just throw it out from his mouth. I don't think he does not like the taste of the food, and also not because I did not make food variation for him. I don't know if I should call him as picky eater. I brought him to doctor. The doctor said that he has problem with food allergic. So when he eat something wrong, his stomach will give strong reaction. And as a result, he will not eat easily. I did follow the doctor's suggestion, but he still does not eat so well. I gave him extra vitamins as well, but it does not really work as well. What should I do? I am really hopeless at this moment.