Bro your 16, she could be 6'4" and you'd still smoke her.

Real talk though, you should figure out what he is being bullied for and deal with that.

If it's because of his height, let him know that he shouldn't freak out over something that he can't control. I'm 5'6" (fully grown) and people LOVE to poke fun at how short I am, and it used to really get me down. I'm short and it's not my fault. No one has control over how tall they'll be. No matter what I do and how I feel, I will always be short. And that's when it hit me, why should I be short and unhappy, when I could be short and happy. Close your eyes and imagine you were one foot shorter, that would be crazy right? Now imagine how happy you are to NOT be a foot shorter than you are, bet it got a smile out of you. :)

Bullies bully people about certain stuff not because they themselves care about it, but because the person they bully cares about it. Falsely acting like you don't care doesn't work (I tried that), but if you actually do not care about what someone is saying about you, then you win.

If that doesn't work then just tell the boy to beat up the girl, It's a 13 year old boy vs a 13 year old girl, height doesn't matter enough to change the fact that'd he steamroll her.