Hey, Bro! Hope everything comes out excellent and my best wishes for you! I had almost the same problem like you: 6'-0" 142lbs since I was 15 and couldn't make anymore than that until 2005. I'm 27 now, and wasn't until I was 25 that I made it to 190lbs. Wanna know the trick?
Getting married!
But seriously, try to go to the gym, and create a body building routine. Not to get you huge, just to tone a little and build muscle. Take as well some protein and vitamins, and you'll get some weight.
Joey