He is underweight by around 3 kg. Dietary issue or reason for child not accepting feeds has to be found and corrected.
There are no medications for increasing food per se. It is mainly food. You may consult online for proper diet chart. And moreover, how to feed your child, there are some tricks. It won't be possible to type everything.
Diet enrichment by things like fruits, vegetables, pulses, legumes, egg and chicken if you are non vegetarian, oats and fiber.
Next Steps
can take consult for better clarity