What should I give my 16-month-old toddler for weight gain?

Hi doctor,

My toddler (16 months old) has had very little milk since birth. The birth weight was 3.1 kg. Now at 16 months, the weight is 8.3 kg. From the eighth month onwards, my kid did not gain weight and was 7 kg till the 12th month. But my kid is as tall as other kids of the same age and makes gestures, walks, runs, and talks perfectly fine. My kid communicates better and understands tasks better than children of the same age. My child is gaining 400-500 g every month now but is very thin. My kid is a fussy eater, and I have to trick my kid into eating but eats a reasonable amount, and I feed my child a balanced diet. I exclusively breastfed (still breastfeeds), yet I tried several formulas before the first birthday as there was no significant weight gain, but the kid would vomit. Now my kid drinks 12 to 14 ounce milk every day. What should I do to get my kid fat? Is there anything wrong with my child? Please help.