Hm, I'd just point out that if fighting is so totally out of character, for both boys, then I'd start at trying to establish what happened.
I think it is a possibility that your DS did not 'choose' to fight, over any other possible approach to the situation, but that it was to an extent out of his control. He knows that fighting is against the rules, and he usually tries, and is successful at, not breaking the rules. So why not this time? Perhaps he tried, but failed this time. It could be behaviour as a response to something underlying. Some big emotions, and he was no longer able to control his reaction. In which case I'd try to address those underlying emotions. And, if he was not able to find a different way to deal with them/the situation, so why expect him to find and write out six different ways now? Perhaps you need to help him find different ways to deal with it.
Or it could be that he did 'choose' - that he basically figured, I want to try this out, see what happens. My similarly aged boy whose behaviour is seen as exemplary at school, has recently really tried 'flexing his muscles' at home, testing the boundaries, pushing the limits. I never used to have to do much in the way of 'parenting' because he always behaved so well, now suddenly I have to step it up!
In this second scenario, what we do is try to have clear, spelled-out rules and consequences. E.g. we never did have to have rules/consequences regarding name calling because it would never have occurred to DS to name call. So now we are in the process of establishing all sorts of rules/expectations and spelling out all sorts of consequences (we try to stick with 'natural' consequences when possible). And sticking to them.
We are also trying to get DS to understand the reasons for the rules, because ultimately we'd like him to stick to them because he wants to, rather than because he wants to keep/earn x privilege.
So in your case that would be a chat (or a series of chats) about why you expect him to find other solutions to conflict rather than fighting. What is wrong with fighting. Is it just about sticking to the school rules, or is there a deeper reason for not fighting. What are consequences of fighting. Etc. In addition to clearly spelled out rules about not fighting, and clearly spelled out consequences. Don't just apply the consequences now, but spell out to him what the consequences will be if he is caught fighting again.