Help for child not eating enough
Though we've never had issues with our children not eating enough, if I did, I'd increase the amount of calories and quality of food they are taking in. First I'd try high calorie healthy foods and if they needed to see if the child would gain weight try some unhealthy foods as a last resort.
High calorie foods could include: avocado, cheese, nuts and seeds, smoothies, granola, quinoa, any nut butters, adding butter, olive oil, coconut oil to foods to increase the amount of calories they were consuming.
It is difficult, but the best thing I have done is to layoff on trying to convince him to eat. We had many more battles over food when it was that way. As much, as I understand any parent worrying about their child's intake- you have to consider the whole picture. The one thing I know is my child will probably never be overweight.