Weight calculation depends on alot of factors
If we assume his birth and birth weight were within normal limits, then his ideal weight should be between 9.5 to 11 kgs
Next Steps
improve his diet, increase the fats and protein in his diet and avoid any form of refined carbohydrates.
fats can be increased by adding ghee or other vegetable oil to the food that he is already taking
proteins can be increased by adding 1 egg per day to the diet
Health Tips
keep a track on his weight, if he loses weight or doesn't gain any weight at all in the next 2 months, better to reach out to a doctor for physical examination.