My baby is 5.6 years old n his weight is just 15 kg what should i give to him for weight increase??

yes you are right, the weight is quite low for this age. you will have to start including healthier options in the foods that he likes to eat. start sneaking in fruits and vegetables in some forms, such as milk shakes, desserts, chilas, sweet paratha stuffed with chopped fruits and such. also add more dairy items and eggs and chicken if he eats non-veg. add ghee and butter to all his meals, and also add raisins, almonds, walnuts and such to foods. give him raagi, daliya and oats as these are extremely healthy for everyone, and particularly at this age.