Assuming your child has no underlying medical concerns, the idea will be to get him to take in more calories. Try to plan healthy, calorie-dense small meals and snacks for your child so he gets enough to eat.
Some healthy high-calorie foods that you might offer your child are- Whole milk, cheese, yogurt, eggs, peanut butter, bean soups, nuts, bread, mashed potatoes.
Consult doctor, he may recommend a high-calorie supplement drink which provide additional vitamins and minerals that your child may be missing out on.
If your child is getting enough calories but still does not seem to be gaining weight appropriately, continue to work with his doctor to discover any underlying conditions.