My ex also has a very fast metabolism and is your height and weight. He could eat 24/7 and never gain an ounce. Ustwo is correct, as well. You have some years of "filling in" ahead that will occur naturally. I wouldn't recommend forcing a weight gain; that doesn't sound healthy.
I grew an additional two inches after I turned 18, and felt a bit scrawny for a while. The curves followed.
