Ahh jeez. Tough age to be these days.
Here's my take on it.
It's 8th grade. All kids get teased somehow. As far as girls picking on boys, what ever happened to the saying that they're only doing it because they think he's cute? Empower that boy so he doesn't feel hurt by their comments. At 6' in middle school, he'll be a star come high school and sports. If he's into that.
Side note: I never got picked on by girls at that age. However, I got picked on miserably by the boys. I was put in school early. Everyone was a year older than me. Wanna know what happened? I hit puberty and grew and beat the **** out of every single kid that looked cross at me. I was not to be messed with and all the girls knew I was the badass and ended up on top.
My point being, he's at a crucial age right now. He needs you to listen and understand him. He's going through puberty. Things are going to be strange no matter where he is. Just give him some confidence and that doesn't mean fighting, since we're dealing with girls.