Skinny guys always think they eat a lot, just like fat people think they don't eat that much. Both are equally in denial.
Log you diet for a couple of weeks on fitday.com. Then you'll have an accurate assessment of what you're really eating. It's probably not as much as you think. Do GOMAD for a month at a time if your body tolerates milk. It works. But simply put, if you aren't gaining weight on your diet, then you are eating at maintenance level. It doesn't matter what you think. AAS are not a magical cure, they are only an adjunct. They will never compensate for inadequate diet or training. Remember, you are forcing an adaptation. Your body does this reluctantly and it will always be a struggle. It would rather be fat and have plenty of calories for the uncertain future.
As for training, is your routine built around low-volume, heavy compound lifts? Are you trying to put more weight on the bar every single workout? Or are you doing some split you read in a BB magazine that has you in the gym 14 hours a week, doing 20 exercises each day? 120lb guys don't need to be doing isolation exercises. Your arms will grow with your back and chest, once you figure out how to make them grow. These are all things you need to consider before you even think about AAS.