Angie, misc.fitness.weights is better suited for this question, please
stop by there and ask for him. They discuss this issue all the time.

The answer is, like in weight loss, diet and exercise.

Eat more, eat better, exercise more.

http://tinyurl.com/35mtx
http://tinyurl.com/3cwjp

i

Tell him not to kill himself.  Tell him to bulk up on high fiber food and to
do weights to build muscle.  Plus cardio or other aerobics for his heart.

Tell him to do what my boyfriend did to me, the big plan. Eat lots of
spagetti, but use fetticinni noodles to make it. Eat nachos and drink
on the weekends. Eat a grilled cheese sandwich, eggs, and a tater
square for breakfast every morning. I used to be 90 lbs when I met my
bf but now I weigh in at a whopping 160. I'm only 4'11". I never had
much of an appetite, until I met him; he loves to cook and feed
people. He's 5'll" with a large appetite and he would make my plates
the same size as his and eventually I caught up to him, and all that
food caught up to my hips. Sad but true.

Angie,

If he's just looking to gain weight, it's simply a matter of calories. He
needs to eat more and eat better, in addition to hitting the gym on a
regular basis so he gains all over, rather than just in the tummy. If his
metabolism is incredibly fast, there are lean mass gainer protein shakes
that come in around 1000 calories each. A couple of those a day on top of
his increased caloric requirement and he will definitely put the weight on
quickly. I have a friend who is a vegetarian and is trying to become a
bodybuilder. He's managed to put on 30 pounds of lean mass in the last eight
months, so it's possible.

Sounds like more fun than dieting, I'll tell you that...

Mike
196/161/Whatever it takes for me to get to 15-19% body fat.