Oh my...where shall I begin...?
Gain weight first before thinking about building up muscles? What
kind of "weight" do you think he'll be gaining if he doesn't gain
"muscle"? Let's see, there's fats, bone density and water. Water
obviously won't help in gaining muscle, bone density can't be
easily increased without strength training and proper diet, which
will also increase muscle mass, ah, so all that's left is
fat...
Are you suggesting he get fatter before gaining muscle, or rather,
are you suggesting he gets fat, and then "convert the fats into
muscles"? 