I just want to address the fact that he believes having others know he has Autism means they'll recognize him as stupid. This is a very real fear among autistic teens and children. The fact that this is a medical, neurological diagnosis means that he has a right to privacy on this issue. If he doesn't want to tell anyone, if he doesn't want you or your mother to tell anyone, then he has the right to keep that information private. If he ever chooses to advocate for Autism Acceptance, he has to be able to decide on his own, OK?
Reassure him as always, but also reassure him that you won't say anything to people that he doesn't want to know. It will be his choice on who to tell outside of medical doctors. When I "talk Autism" with people in real life, if my daughter is present I ask her for permission first. Then I ask her for permission to use examples about her. If she says "no" then I abide by it. She's only 9 1/2. I want her to feel some control and self advocacy.
Okay. So now the meat of what I want to address. Right now, it's become common for teens and young adults in real life and in movies and on Facebook make jokes about Autistics. The jokes are insulting. People use "autistic" in place of "idiot" and "stupid" and worst of all, "retarded." If someone does something socially awkward or unusual, a common insult is, "What are you, autistic?" If he's 18 and only recently graduated high school he probably heard this sort of thing on a daily basis from other students who don't know any better. Students/people who don't know what ASD actually is.
I would love it if you could suggest to your brother to research Temple Grandin and maybe find him some of her books. She has ASD... her form of it is Asperger's Disorder. She's wonderfully articulate and is very active in the ASD community.
There are also many ASD blogs out there written by people who have ASD. It might help him to know that he's not alone. Landon Bryce is one such individual. He's also on Facebook.
The more your brother learns about his own disorder, the less ashamed of it he may become. Knowledge is power... and the more knowledge he has, the more self esteem he'll develop.
Show him famous people and geniuses who have and are suspected to have ASD. That'll really blow his mind. :-)