Gaining

hehe god luck but can´t help you really cause i always had the same kind of metabolism ;-)

Usually people who eat junk and don't gain weight are inconsistent eaters. Do some major stuffings, have him drink a lot of beverage (even water) between meals to keep his stomach stretched, and have him carry shakes, trail mix, and snacks around at all times. He's gotta consciously eat and eat for a few weeks (maybe even more than a month) so that he gets used to eating a lot consistently. After that training, he should be able to eat a few big meals a day plus snacks.

And then you can get him as fat as your heart desires. Good luck!

^^Very true. But metabolism is sometimes a major sticking point. The quickest way to slow down a rapid metabolism is to crash-diet. (Not fun)

I suggest you try Music's idea first though.