He's not even really that underweight, to be honest. The dude's just not done growing. Hell, I was 5 inches taller than him and the same weight in high school.
OP, you're making a HUGE mistake by going this route. Please don't do it. I'm sure you're a complete idiot, and you probably won't listen to any good advice, but at the very least, make sure you're done growing before you make this decision. K thanks.