Tell him to sing every day. It will build up his voice and the muscles he uses to produce the sound. An athlete wouldn't just start running a marathon - he/she has to train, and it's the same with the voice.

Also make sure he is singing in the right key for his voice to produce a reasonable sound, with strength and volume. Too many people have what I call a bedroom voice. They practice in their bedroom, and it's all very nice and comfortable, but when they get into a bigger room, they can hardly be heard. The solution is to knock the key up around a full tone.