My brother, 21 yo, is like this. He’s ~180 cm tall and has never weighted more than 61 kg (during the 2020 covid lockdown, he made a big deal out of this since it was the first time he passed the 60 kg mark; after the lockdown was over he immediately dropped back to his perpetual weight of ~55kg). I live with him so I know his eating habits: he eats all the time, whatever he wants, whenever, how much he wants and simply doesn’t put on any weight. Is very energetic, no sick look / demeanour. It’s just fascinating to me cause I’m not like that at all, I have to watch what / how much / when I eat.
As usual, weight is driven by Calorie in - Calorie out. If it’s positive you gain weight, if it’s negative you loose weigt.
Your brother is a tall, young male, all of that would point toward a relatively high calorie out also without knowing him in details it’s hard to evaluate. There is always that person taking desert twice when out, but skipping lunch on regular day while doing a lot of sport.
this is actually misinformation that’s already been debunked. I recommend reading my other comment thread