"oh, okay." is generally a good bet, then if the person wants to they can expand on any special needs they have.
Like if someone's physically disabled you'd be rude to make a big deal out of it, you just acknowledge that it is what it is and try to accommodate for it.
"Oh cool! Are there any particular triggers we should know about, like loud noise?"
Autism and its effects can differ greatly from person to person, such that the 'rules' for dealing with one person might not apply to another.
In the context of revealing your autism, many will not do so simply out of hand because of insert response from OP's post. Usually the only reasons many high functioning autists will even reveal it to someone they do not trust is because they either have to (eg: there is the potential for the presence of a meltdown trigger, reasonable accommodation requests at work) or they're relating to someone with their own disabilities.