My one year baby's height is 29 inches and weight is 8kg. Is he underweight? If yes, wot should we give him to put on weight.

Hard to do. First it makes a difference if it is a girl or boy. But 8 kilos is less than the 5th percentile. So what realy matters is has he always been below but his month to month growth is normal. Talk with your pediatrician and review the things you may be able to change in his diet.
Calories. Beyond specific foods, which are culturally different for us all, the main ingredient for weight gain is calories. High density calories are usually those with higher fat content, such as whole milk, cheeses, sauces and gravies. This is not an endorsement for lifetime eating habits, but specific for gaining weight.