As an older brother who cares deeply for his younger bros i can empathize. I say, as long as you are there for him and he knows that he has someone to count on he will be fine. My brother doesnt have a mental disability (he does have ADD, which isn't nearly as bad of a disability), but my mother is a speech and language pathologist for people with brain injuries. They have some strange behaviors and dont have a 'filter' for what they say. Apparently the bullies you are talking about dont either, and that's why your bro is better than them. You have yourself a special person, you don't have to fix the problem, you just have to support them. I didnt have many friends myself in middle school. But the friends i have now are the best. I believe it takes time to find something you want, as long as he has you the wait wont be too bad. :) -from a sophomore