You asked

What can I do to help my child gain weight?

There are many factors to consider when deciding if a child is truly underweight. First of all, if there are any underlying medical conditions, those must be considered. Second, genetics plays a major role. If both parents are very thin, the child will more than likely be thin as well. And third, if the child has recently gone through a major growth spurt, they may have thinned out, which is normal. Their body weight will probably catch up to their height soon enough.
If you feel that your child is too thin and needs medical help, see a doctor. They will evaluate your child’s weight by considering the above mentioned factors as well as their place on the standard growth chart. To make this determination, a child’s body mass index (BMI) will be calculated. If your child’s BMI is below the 5th percentile on the growth chart, they are considered underweight.
A doctor may then run tests to make sure that there are no medical problems causing the weight problem. Once they have determined that your child is healthy, they will more than likely have you see a dietician who can direct you on how to properly feed your child to allow them to gain weight the proper way.

More questions

Trying to feed a picky eater can make mealtimes even more stressful. While food fussiness is a typical phase that most chldren will through, it doesn’t make it any easier for mums to deal with. However...
Foods on this shelf are the best energy providers for your child's body, so the more active your child is the more he will need.
There are many nutrients found in whole-grains which are essential for your child’s health.
Check out these simple steps you can take to make sure that your child gets enough calcium.
Dairy products such as milk, yogurt, and cheese are some of the best sources of calcium, but it can also be found in less expected sources.
There is a difference between iron that is found in animal sources and plant sources of iron.
Iron is vital in order to ensure good health and development in your child. It is responsible for making haemoglobin, the pigment which carries oxygen to the blood and myoglobin, a pigment that stores oxygen in...
If your child rejects a lot of vegetables, there are lots of ways you can try slipping them into food. 
What exactly is cholesterol and is all cholesterol bad?
There are many factors to consider when deciding if a child is truly underweight