Go to his school sometime, hang out with him. If bullies see you hanging around him, they will stop. Heck, even go threaten the crap out of the bullies or cuss the fuck out of them. Tell them you will beat the shit out of them. Don't tell them it is because of your brother, just tell them you think they're annoying as fuck, and that you just need one more reason to hospitalize them. If they don't buy it, tell them to watch their back because you will be there when nobody is watching. If nobody is watching, do something that will not incriminate you. Come up from behind, and make one of them black out. They'll back out. Be a bully to them, and they'll stop being a bully to your brother. But you don't actually have to do anything. Threats themselves should make them back off.
If threats aren't your thing. Then talk to him. Say something along these lines.
"Are you curious what death is like? It's nothing. No thoughts, no dreams, no heaven, it's nothingness in a box buried six feet underground. Okay, so let's say you kill yourself. If you do, you will get what you want. People will miss you, and people will wish they treated you better. You will win. You won't have to deal with anyone else for the rest of eternity, even the people who love you. But our parents and I will be devastated for the rest of our lives. Our lives will be ruined without you because we all love you, and I am your older bother. It is my job to protect you UNTIL I die. You may win, but I will lose. When your gone it won't be long until everybody will forget about you. You will known as the kid who was too weak, and quit living. Do you want to be known as the kid who was too weak? I need to tell you something imporant before you make a choice. You have your entire life ahead of you. Let me tell you something else. You know all of those bullies? Once they graduate, let me rephrase that, if they graduate, they won't get very far in life. It is the people like you, the one's who stick through all the pain and suffering, that are the ones who come out on top, the ones who truly win.
Aren't you curous about what it is like to be in love? It is the most amazing thing any one person can ever experience. It will bring joy to your darkest memories. Love will make you laugh and smile. You will feel like the luckeist man in the world. To be honest, I can't wait till I am in love (if you haven't been yet). You are my brother, and you will always be my one and only brother. I want to see you grow up. I want to see you fall in love. I want to be an uncle one day. Please, please, don't leave me."