Hi,

My son is 6 years old. His weight is 18-20 kg and height is little below 4 fit. He looks very small.

How can he put on weight and height.

Allow him to play outdoors and develop his large muscles on playground equipment, bikes and trikes, ball throwing, roller skating, badminton, etc. Look at the males on both sides of the family. Are they under or over six feet tall, or a mix? Your son is a "victim"" of his genes for height. Most boys go through a growth spurt at puberty and after until about age 21.