May be I am not an expert, but I have an experience in this topic.
My sister had an underweight child, she would feed him so much but he could never gain weight. She would call me up crying at night telling me how her son would get picked on at school because of his underweight. She felt like she tried everything that she could think of and nothing worked.
I thought to myself I could maybe help him gain some weight. I called up my sister and told her I can take him out grocery shopping with me, to see what he likes, and then try to incorporate some healthy food to help him again weight. She agreed and was extremely excited about. Maybe I will be the answers to her prayers.
I took John to our local grocery store. I was straight forward with him and told him the plan. He was onboard, as long as I didn’t tell anyone he knew about it. I agreed. Not to my surprise John loved protein and he told me when grabbing eggs that he likes the white part but not the yellow. He loves steak, pork, and chicken; by the end of shopping I noticed the majority of his food was all protein and no carbs.
I pointed out his diet, so he knew what he was eating. It was just one branch of the food chain. I informed him protein will help him build muscle but not again enough wait without carbs and fiber. It was my turn, we started in the veggie isle and I started grab potatoes, greens, eggplant, and squash. Informed him not all carbs are good, I suggested to eat whole grains like, brown rice, whole grain bread, brown pasta, etc. I threw in some peanut butter because I know it is high in fat and can be good on apples, celery, bread, etc.
I wanted to make sure that he knew how to gain weight; it wasn’t about eating junk food. It was about eating healthy and having a wide range of food on your plate. Not just sticking to one section like protein. It is a life style that he will have to embed it into his life. I told him I know it will be hard but myself and his mother would be with him every step of the way. About 2 months later, he had again 10 healthy extra pounds and has been maintaining his weight ever since. I am so proud of him. He is now able to walk down the hall of his school without any children making fun of him.