Don't listen to this guy. If anything, 2 had it on the dot stating that this is a dominance issue. If you try to settle this in the way 5 told you to, it'll only make you look weak and you're just trying to appease your aggressor. Of course, this doesn't mean be a barbarian and bash his head in with a chair or something. Instead,just physically resist him. You state that because you're much smaller than your aggressor, you tend to not fight back. It's the very same reason that because you don't retaliate in any way, it makes your aggressor feel like it's ok to continue to do something like that. So the next time when he does something like this, just fight back-although you probably won't win, it sends a clear message to him that you are willing and are capable of fighting back so at least, the teasing should ease up.
In my own personal opinion though, you sound like a godamn nancy boy. Seriously, I know that this is a personal problems board, but Jesus H. Christ I can literally hear your whining through the internet. Just reading your post makes me want to beat you up simply because you present yourself in such a girly and weak manner. As 2 said, grow some balls and man up. You're 21 and you still refer yourself as a boy? disgusting