There absolutely needs to be some consequence for punching his brother. then again, it isn't the end of the world. Brothers do that sort of crap sometimes. What seems to be missing is knowing what set him off. "Nothing," isn't the answer, unless he is just a cruel guy to begin with, and that is a totally different topic.
As for moving forward, be the same consistent fellow you have always been. To kids, up to about 17 years, parental consistency is the foundation of security and safety in their world. And to them nothing says, "I love you" as loudly as rules, reasonable punishments, and the parental effort to follow through.
There is no light switch transition between boy and man. So he took a few steps back. Eh! Point it out, let him know you expect better, and start over (isn't so much of parenting starting over, and over, and over again with our kids till they get it?).
Probably one of the hardest things for parents is to tell their children they love them, without condoning their behavior all the time.
"I will love you even when you act like a jack-ass," is a great affirmation!
A little different from the DS, approach maybe, but I've found it's very easy to lay that on young boys, (this idea of being a man) which is probably stressful. Stressful for us dads, since they will inevitably fall short and stressful for them since they are just kids. I would make sure that I was doing more "passive teaching", being available for him/them for them to express themselves without fear of punishment etc.
I have three kids myself, though only one boy. I'd highly recommend these books, the second I am just making my way through now.