I think the mom would totally understand if you asked her directly how to do this, or if your gf did.

Yup, you or your g/f should ask his mom and preface it the same way you did here: in terms of making him feel welcome and not wanting to unknowingly freak him out.

The 'personal space' social issue can vary widely in autism... Some people are bothered by having their personal intruded on (for lack of a better word), while others sort of unknowingly intrude (ditto) on the personal space of others. Like holding a hug too long, stuff like that.

It really depends a lot on his 'degree' of autism, and how much (if any) and what kind of therapy/rehab he's had.

Yeah the Mom is the right person to give you cues on this. I know a few young people with Asperger's type autism and they can hang out with other people pretty much normally except for being very very fussy about things [alignment of food, speace issues, etc], at least in the short term.

I have a buddy with Aspergers and he says the most helpful thing for him is for requests and statements to be made as straightforwardly as possible. He's not good at reading cues, but he's happy to just take your word for things, so if you say "hey you're standing a little close to me, could you back up some?" it's a non-problem and he never thinks of it as a rude request. So, thinking about what you're saying and not just how you're saying it is important.

Obviously this varies tremendously based on the person, but being direct, asking in advance about things like food and space issues and just trying to be inclusive without being etiquette police about it are likely to be good starts.

Definitely ask the mum, he might be non-verbal! Also ask about idiosyncracies, fears or phobias it would be good to know about beforehand (such as direct eye contact - some people with autism find it very uncomfortable) or specific types of sensory stimulation he finds overwhelming (fluoro lights, or some smells, for example). His mum will also be able to tell you if he has a specific (obsessive) area of interest you can start chatting about to build rapport.

Otherwise, just treat him as normal, but be aware of the language you use (clear, simple), your tone of voice and body language (many people with autism are easily able to pick up on insincerity, for eg), don't worry about it too much and enjoy yourself!