i got the same prob im 16 5 ft 9 and only 115 lbs even if i eat loads of fatty stuff i cnt put weight on iv being doin weights for 12 months now and my arms are a bit thicker and quite hard but thts it the rest of me is still quite thin but for tht weight id be flyweight when i strt competin later this yr but i got a 67.5 inch reach so is tht an advantage