Autism isn't a terrible affliction you have to dread, so don't let him think it is. He will not be autistic from diagnosis onwards, he's always been autistic, he's the same boy he was before the diagnosis.
He doesn't have to tell anyone and neither do you. You can inform school and make it clear it's his confidential medical information and he does not want anyone else to know.
Would he be happier with the computer analogy, NT people's brains process things as though they use Windows which is really ordinary, whereas autistic peoples' brains process things like a Mac which is a more unique and far and better processor.
Stress, stress and stress some more, different does not mean inferior.
This also is more positive about strengths, read it first, it may be a bit long for him, I think there's a youtube version too. the-art-of-autism.com/understanding-the-spectrum-a-comic-strip-explanation/
Find a list of very clever and successful people who are or who are thought to have been autistic. The more positive you are about it, the better he'll feel in himself.
It is hard to hear a diagnosis because it only focuses on the negative, give yourself time to process that, then look for a way to put it to him in a positive light, in a way you know will make him feel better about it. 