Why my toddler is skiny even he eats alot?

My son is 17 months old and he is so skiny. His birth weight was 8 pounds. He now eats everything, vegetables, fruits(alot), grains/porridge, meat, drinks cow's milk.

He is a kid that needs everytime to eat like he is eating with me, then sees someone else eating and starts to cry that I want to eat with you as well lol. Like I'm so satisfied with his eating alot habit but the concerning point is, not gaining weight. His ribcage is easily seen when naked. So flimsy is he. He poops 2-3 times and sometimes 3-4 times.

I want to know mama's around the world, did they ever faced this thing with their toddler? Or any opinion?

Thanks