[JustACityBoy]

I don't necessarily have much advice for you, however, I can tell you a perspective. A girl who I'm pretty sure likes me knew I was being bullied.
I was just like your brother, I told myself, I could handle this. Learn karate, learn martial arts. Kick his ass next time he touches me. However, I was weaker then him, but I probably could injure his ass enough to the point he wouldn't touch me. Win, even, 60% chance. Quite confident in my punch speed and strength.
However, I didn't do that.
One day, it was gym, a douchebag and D told me to go in the lockers. Douchebag was actually my friend... listened to my problems and laughed in class, so of course I went in, we did some push ups to see who was stronger. D however, was like his little dick, constantly following him. He pushed me to a wall, and it didn't hurt, honestly, but the girl saw and she told him to back off, or else she's calling the principal.
Basically, it made me feel week. I said, "I was fine". Was I though? Not really.
To be honest, I never wanted help.
I think I couldve solved it... well, the result is me right now. Might be bad but..
This was when I was 13.
This is merely from perspective. However, I know one thing: That year sucked each time he was around, and I'm sure he was a factor in why I'm so anti-social now.

You should help him if you think he can't handle it. The emotional effects really hurt.
Otherwise, if you do help him, people are going to think he's a wimp. (again, totally my perspective and how I felt.) and cant defend for himself.

Edit: I forgot to add this very important thing. I shouldve defended myself. I feel crippling anger each time I remember I didn't.
My first act of defense was to push him, but it went bad, cause I lost my nerve, If I continued, however, we would've had a fight, and well, again, pretty confident. I was learning moves from a guy who sat next to me. Pretty good at them too.

So now, I see myself as a sheep. Fcking shouldve punched him...
However, it made me realize this: Dont let this ever happen to yourself.. EVER. Again! Bullies only prey on the weak, who don't retaliate. I saw him do the same to some skinny ass guy for some money.. I felt pretty bad seeing that and not doing anything. Well, I was scared of him and I kind of hated that skinny guy, he kept saying I had no friends in an angry way, and always called me a nerd, in an angry way. He wasn't a hotshot himself.

Currently, I HAVE been bullied once this school year. BIG guy, tall, muscular, chubby though, drew on my arm cause my fricking weird friend drew on my arm too, (I didn't allow it, but I never showed anger, so he kept drawing weird ass scribbles), and he drew on me too. I said back off man, he continued to do it, but the 3rd time, I pushed his hand, and he stopped. I am small too.. 5"3, some muscles visible.

edit: I just realized I wrote a novel. Sorry.
Summary: Helping him will make him lose his pride, and his manliness. However, if it's so serious to the point where he's hurt in the future, you have to help him.