My son is 18 mths and also around 9 plus 10 kg only. Is your son very active, like walking, climbing all the time? If he is, probably there's no extra calorie left to gain much weight? So long as he's healthy, should be ok right?
Have you let him eat rice already? My son doesn't like porridge from 15 month onwards so I moved him to rice usually with soup and ingredients like meat, fish, veg, tofu, egg. In fact, he's eating the family meal now (whatever I cook, I use less seasonings), and outside food once or twice a week. Is he self-feeding? Try let him self-feed, he may find it fun to eat. Messy but my son likes.
His schedule is as follows:
Breakfast - fresh milk + cereal with avocado/bread/cake/oatmeal/pancakes
Tea - fruits like apple, pear, banana, watermelon
Lunch - rice with soup and ingredients
Tea - fresh milk/yogurt + pancake/muffin/bread/cake
Dinner - rice with soup and ingredients + fruits like grapes, dragonfruit, plum
Before bed - fresh milk if he wants
In between meals, sometimes he asks for biscuits, I will give if it's not too close to meal time.
Sometimes I give him noodles, pasta but he doesn't like, he's a rice person. The rice I cook is half white rice half brown rice.
There are days where he doesn't eat much too, I get upset and try to coax him to eat more, but let him go if he really doesn't want.