Don't worry, dude. I was like one of the shortest kids in my grade in 7th grade. In 8th grade, I was about middle of the pack. Now I'm like one of the tallest kids in 9th grade. I'm only 120 pounds. My cousin didn't hit 100 until 9th grade, though, and now about 4 years later he's hovering around 180.

You probably have really high metabolism like me, which means you can eat as much as you want and you won't gain weight. I eat a 10 oz steak at least 3 times a week, sometimes 4. I eat tons of cookies, donuts, drink Mountain Dew heavily, and I'm still 120 pounds. Give it time. We'll fill out one day. I'll probably be some bald fat dude in 10 years because I eat so much.