I've found that kids don't really use our kid's forum. Probably because thoughtful conversations simply are not how kids interact with each other. My 10 year old daughter and her friends email each other all the time, but it's all silly stuff, and rarely more than 7 nonesense words. In person, the conversations are play centered and jabber jabber. She'll interact with kids on places like Wizard 101, but that, too, is silly. It seems to be how kids relate and connect.
My AS son ... totally not interested. Except for on-line games. Texts with his friends but won't read his email.
So. I don't know to help connect your brother with others like him. Any gathering place for kids would have to be heavy on the games and fun stuff, and light on the "make conversation." That may not seem like it would serve much connection but, for kids, that's how they do it.
It's also safer for kids that they don't make conversation on the internet, in general, so I'm kind of wary of changing the pattern.
Although ... there could be an article section with very short quotes and articles, where the kids can post short respones like "I relate to that!" or "it's not like that for me." That would give them the feeling that there are others out there like them without them having to expose themselves, if that makes sense.
Hmmmm ... that could be an interesting project. A small play oriented companion to WP.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).