Basically, in my opinion, don't have him punch the guy at all. Just have him stand up to him, even if he throws a punch, i find it's better to take a punch than throw a punch, shows you're resilient enough to handle whatever they throw. But that being said if it does continue, go to the larger guy, stand up to him, or go to the actual bully when he's by himself, and try and find out what is his actual problem with your brother. If he doesn't say or it's a stupid reason, and it continues, tell your brother to kick the living daylights out of him.
But if you're really concerned for your brother try hanging back and picking him up from school and taking him, so you have a general idea of what happens, or try and stick by your brother as much as possible without making it look like your brother is "scared"
