Now is time to start using all the dirt on him you've picked up. Start spending less and less time with him. Start being cold. He's going to get neurotic as fuck and panic, because he thought you two were tight. Eventually he's going to single you out and ask what's going on. That's when you lay into him with the armor-piercing, emotionally damaging shit that you've been holding onto all this time during the personal chats. Make him feel like shit. Make him feel like it's his fault, and that you're tired of being around a person like him. When he gets all clingy and panicky, beat him up. Nothing major. Avoid the face and other exposed parts so that when he goes home later, he'll be too ashamed to tell him mom what happened. Anyway, it's going to totally catch him off guard. Even if he wasn't a coward, he might not have fought back, completely caught off guard. The combination is going to leave him speechless, more likely than not.
So this is where you make the decision of whether or not you want to leave him broken, or pick up the pieces and finish the mold.
If you want to keep him broken, just end it right there. Don't talk to him anymore, and make it clear with body language that if he comes near you again, he's going to get whooped. Talk mad shit about him to your friends. Every dirty detail you picked up, and add more later that are exaggerated. He'll be done for. He'll never trust anyone again, or at least not for a very long time. The confusion alone will drive him mad. He might try suicide, but more likely than not he'll be a fragile husk of his former self, which is about as broken as he can get without real, implicating trauma.
If you want to build him back up, as soon as the fight is over, make it very clear that you're sick of the fact that he can't stand up for himself. Your friends have been mean to you and made fun of you for hanging out with a pussy. You're ashamed to be around him, and even though you like him, it's putting a major toll on you. Now, he's going to feel like shit, because just someone trying to be his friend is getting them picked on. Tell him, if you could just stand up for yourself, the torment would end. He's going to take the bait. Tell him the next person who gives him shit at school, he needs to attack. You might have to teach him a few little things about fighting, but people who've never fought before are a letter better off running on their instincts. They'll be more savage and scary that way.
After he fights back, win or lose, let him know things are cool, and that you're proud of him. Have him join your circle of friends and make sure he doesn't pussy out anymore with bullies, even if it's people you know and like. They had it coming after all.
So, in probably less than a month, you've taken a dorky nerd with no social skills, broken him down, and turned him into an acceptable friend/minion. He's going to consider you his best friend, maybe for life. The guy who made high school no longer living hell. You won't have a more loyal friend, or a more willing accomplice for pretty much life, as long as you keep contact up over the years.