Well, it is not as easy as “he just needs to eat less”. Yes, if you starve a person they will get thinner.
But from what I have been hearing, the state of science today sees genetic predisposition as a much bigger factor than previously assumed. It also affects how the feeling of satiety works in a person. What many slim people cannot wrap their head around is how another person, eating less, might permanently feel hungry.
Again, I’m not that deep into the science of it, that is just the general gist of what I’ve been hearing.