Teach your brother about trolls. Honestly, you can't avoid it or do anything against it. The sooner your brother learns about things like those, the better for him. Trust me on this one. Maybe it was even for the better that he experienced something like that. I'm not saying that this should happen to him or anyone else on a regular basis, but people learn how to deal with certain stuff only when they experience it themselves. This is what allows us to grow and to continue on our path.