Swift and immediate punishment. Remove him from the situation, explain any violence is inappropriate, then have him sit in a time out - by himself, in a corner, in a quiet room is absolutely necessary. If he refuses to sit, sit in a chair with him and hold him there.
My advice is do not give him three chances, do not give him any chances EVER. If he behaves like this, he is done. You have to be firm about it.
Do not yell or carry on or act like he is. It will only make him do it more as he's seeing you as an example to follow. Just grab him - harshly if you have to - sit him down and tell him he is NEVER, NEVER, NEVER allowed to do that again.
Also - I would start a reward system. For example, if he goes one full day without behaving mean towards his brother, he gets a piece of candy (or a treat) after dinner and so does the brother. "Both of you are such good boys!"
If he misbehaves, he gets a time-out and neither of them get candy.
Soon, he'll start getting pressure from the older brother to behave well also because both of them want the candy treat.
Try the reward system for one month and see what happens - I've seen it in action and am AMAZED at the results.
Good luck!