Obviously there's no one-size-fits-all solution for young adults with autism, and clearly you shouldn't be put in the position of having to make such decisions about your brother, but I'll offer my brother's story in case it has any elements that might help at all.
When my (also autistic) brother was about to finish high school, my parents realized that by having him not take the last required class and having him get a Certificate of Completion rather than a diploma, they would be able to keep him in government-funded school activities until he was 21. So he went to this "transition program," designed for young adults with disabilities, which offered some academics, some art, and some vocational training. Because he was there, our regional center also funded some after-school activities for him.
Now, he's 22, and he went back to high school to complete the last class and get a diploma; in Fall, he starts classes at a community college. About a year and a half ago, he moved out of my parents' house and into a group home (funded by his SSI), to learn some independent living skills that he wouldn't have been able to learn at home, with my mom picking up after him everywhere.
Like your brother, he will likely need some government assistance for the rest of his life, but he's attained some degree of limited autonomy, which is the best that we can really hope for, I think.
As a side note, I understand the weird quasi-third-parent role that sometimes you're put in as the sibling of a child with autism; my parents tried to deny it for years, but when they conserved my brother, they named me as next-in-line (that is, should something happen to my parents, I would literally become my brother's legal guardian).