Under weight or poor weight gain

Hi Everyone, I am just worried about my son's weight. He is a 20 month old boy and weighs only 8Kg (below the WHO growth chart range). Earlier he used to eat less but now he is eating fine but still there is no increase in weight. My doctor couldn't diagnose anything except to treat his constipation, which is now resolved and made him eat better compared to previous. But still there is no weight gain. Does anyone else experienced the same or have any thoughts/suggestions? Thanks!