It depends what he wants to do, if he wants to gain weight due to personal reasons he should take the bodybuilding route and lift heavy weights and eat 6 big meals a day.
But if he's an amateur boxer, i dont see why he would want to gain weight....Strenght would be the more beneficial way.

Latverian, im not sure about weightlifting being a non benefit to boxing.
Ricky Hatton lifts weights, and he's very explosive in the ring so i dont think the weights affected his speed......Ive heard mixed opinions, but i would rather have a ripped defined physique due to training than a soft looking physique.