Does your child have any food allergies or digestive problem? Reason to ask the questions is that we should be informed about the cause of problem.
However, you can easily solve this problem by giving your kid high calories and nutrients rich foods like cheese, flaxseed oil, fruit smoothies, yogurts, cream soups, whole grain cereals like weetabix, nuts, dried fruits, seeds, vegetables, fruits, and protein like chicken, fish and red meat. Trick is to make it fun look wise and delicious taste wise. It will take sometime for him to get a hang of it. All the best!