If the body itself is made in such a way that it doesn't store fat then it will not help.I said eating in a surplus is a necessity for gaining weight, where you get those calories from don't matter once you are already in a surplus anything you eat will make you gain fat.
to gain weight in a healthy way he must EXERCISE regularly and eat a good amount of protein with a slight caloric surplus say 200-300 calories. This will ensure he gains lesser fat and more amount of muscle.
I know of a cousin who eats like pig and is still thin. His own brother eats the same way but is fat.
So we can't just say eat to put on weight. Also exercise means it should be weight training added with good protein intake then it can help.
If he ends up doing cardio, walking running that will not help his weight gain.