You just need to keep packing on more mass. Its not really possible to only increase the size of your arms. You need to put on mass everywhere, and they will fill out. At least that is how it works for me.
That's not necessarily true.... when I was 16, I was much leaner, had no shoulders, chest, or forearms. However, people would comment on my biceps all the time. I can lift a lot more with my biceps, but they don't look as big. I don't know why, perhaps b/c I have more fat.
Another point. I started doing reverse grip forearm curls again on Saturday. I stopped doing them I don't know when. They not only work the other side of the forearm, but also part of the biceps. They feel awesome today, and my chronic biceps aches are gone!!!! I've had this on and off for years, I'm certain the reverse forearm curls have solved it.