Hi Dear,
since thyroid tests are normal and the baby is otherwise healthy, you need not worry. It is the body constitution that some children are thin. Height is also genetically decided and some children are naturally smaller in height than their counterparts.
Do not try to force feed your child with the intention of making the child put on weight. It will be counter productive as the child will develop a hate attitude towards food. Best is to encourage the child to have whatever food he enjoys eating. Do add vegetables to his diet.

hi
he is under nourish only ,he has no other problem.
To gain weight you need to consume more calories every day from healthy foods and choose the right type of exercise.
Underweight can occur due to eating disorders, chronic diseases.
Underweight individuals are more prone for heart failures, osteoporosis, fertility problems, and cancers

give him following food item to gain weight

Milk group: Milk, cheese, yoghurt, cottage cheese, ice cream.
Meat group: Meat, fish, poultry, eggs.
Vegetable group: Dried beans, peas, nuts, potatoes.
Fruit group: Fruit juices. Fruits like bananas, mango, chickoo.
Grain group: Cereals, breads, rolls, pasta, pancakes, rice, muffins